(dissenting, with whom Rubin, J., joins). I respectfully dissent because I conclude that the jury instructions were inadequate and created a substantial risk of a miscarriage of justice. I would remand for a new trial.
*548The commitment of a defendant to confinement, possibly for the rest of his life, is a most serious deprivation of liberty. The Legislature has therefore instructed that determination of sexual dangerousness, and need for secure confinement, requires a unanimous jury verdict and proof beyond a reasonable doubt. G. L. c. 123A, § 14(d). There is no dispute that jury instructions were required, and some instruction was given.1 However, the jury were not instructed on an essential element of the Commonwealth’s proof, namely, that they were required to unanimously find, by proof beyond a reasonable doubt, that the defendant was “likely to engage in sexual offenses if not confined to a secure facility” (emphasis added). G. L. c. 123A, § 1. “Sexual offenses” is a defined term covering only certain conduct. Here, determining which of the behaviors that the defendant was likely to engage in might constitute “sexual offenses” was left to the uninstructed imaginations of a jury of lay persons.
This case is directly controlled by the reasoning in Commonwealth v. Walter, 40 Mass. App. Ct. 907 (1996). There we considered jury instructions that failed to define the word “felony,” which “is an essential element of the crime proscribed by G. L. c. 266, § 18, breaking and entering in the daytime with intent to commit a felony.” Id. at 909. In that case it was critically important that the jury understand the meaning of “felony,” and the types of crimes covered by that category. Ibid. “Because the jury was not informed of the meaning of felony, ‘[t]he defendant’s fate thus turned on a layman’s definition of [felony], [This is a] technical matter[] with which laymen cannot be expected to be familiar.’ Commonwealth v. White, 353 Mass. 409, 425 (1967), cert. denied, 391 U.S. 968 (1968). See Commonwealth v. Benders, 361 Mass. 704, 708 (1972); Commonwealth v. Claudio, 418 Mass. 103, 117-119 (1994) (judge’s failure to define ‘felony’ in combination with other *549errors required reversal); Commonwealth v. Fuller, 421 Mass. 400, 411 (1995) (judge should explain meaning of technical terms where meaning is obscure and there is a possibility of confusion).” Ibid. (Emphasis added).
What an average lay juror may consider to be a sexual offense is a matter of considerable speculation, and varies from person to person depending on both subconscious and conscious influences, such as family, religion, education, culture, and other life background and experiences. The Legislature, however, has catalogued which specific sexual offenses constitute a sexual offense within the meaning of the statute. G. L. c. 123A, § 1. The Legislature did not include reference to all criminal sexual behavior and clearly intended that the jurors not self-define the term “sexual offense” for purposes of sexually dangerous person (SDP) proceedings.
In this case, the defendant, on five instances during a period of six years while he was in State prison, engaged in exhibitionism either by way of exposing himself or masturbating in the presence of one or more (usually female) individuals. Such conduct may be unlawful under G. L. c. 272, § 16,2 but violation of that statute was not a “sexual offense” within the statutory definition in effect at the time of these proceedings. G. L. c. 123A, § 1, as amended through St. 2002, c. 492.3 There was other evidence of unlawful sexual behavior by the defendant, consisting only of descriptions of the defendant’s initial rape and attempted rape in 1983 and the accompanying police reports. But the Commonwealth’s experts failed to specify which of the sexual offenses that the Legislature defined in the statute they believed the defendant would commit in the future.
“A judge should instruct the jury fairly, clearly, adequately, and correctly concerning principles that ought to guide and control their action,” Mahoney v. Gooch, 246 Mass. 567, 571 (1923); see Commonwealth v. Batchelder, 407 Mass. 752, 759 (1990), and is required to set out the elements of the offense in the charge. See Commonwealth v. Reilly, 5 Mass. App. Ct. 435, 438 (1977). This is an absolute duty that must be performed *550irrespective of whether such instructions are requested by either party. See Mahoney v. Gooch, supra at 571; Commonwealth v. Martin, 19 Mass. App. Ct. 117, 120 (1984) (“[e]ven apart from the requested instruction, the judge had the duty to state the applicable law to the jury”). Any instructions must be based on the evidence as presented at trial. See, e.g., Commonwealth v. Leftwich, 430 Mass. 865, 868 (2000) (in order to warrant submitting theory of joint venture to jury, there must be evidence sufficient to permit finding of elements of joint venture beyond a reasonable doubt).
Here, the judge identified the three principal elements of a c. 123A finding, but did not, and was not requested to, explain the words “sexual offense.” While it is axiomatic that words that retain their everyday common meaning usually need not be defined, “a judge should explain the meaning of technical terms where their meaning is obscure and there is a possibility of confusion.” Commonwealth v. Allen, 54 Mass. App. Ct. 719, 724 (2002).4
In the SDP commitment process, the words “sexual offense” do not have an everyday common meaning but, rather, have been specifically defined by the Legislature. G. L. c. 123A, § 1. The judge should have provided an explanation. See Commonwealth v. Walter, 40 Mass. App. Ct. at 909; Commonwealth v. Allen, supra at 724 (requiring explanation of the term “telecommunication services”). The failure to do so “required the jury to speculate in reaching its decision.” Commonwealth v. Niziolek, 380 Mass. 513, 527 (1980). See Commonwealth v. Kessler, 442 Mass. 770, 777 (2004) (requiring definition of “recklessly” where “there is the possibility that the jury would not realize that recklessness requires more than mere negligence”). Compare Commonwealth v. Fuller, 421 Mass. 400, 411-412 (1995) (no error in failing to define “mental disease or defect” in instruction on criminal responsibility, where instruction followed language approved by Supreme Judicial Court, and where defense experts “discussed the psychological factors in depth”).
*551I next must consider whether the error created a substantial risk of a miscarriage of justice.5 In so doing, “[w]e consider the strength of the Commonwealth’s case, the nature of the error, the significance of the error in the context of the trial, and the possibility that the absence of an objection was the result of a reasonable tactical decision.” Commonwealth v. Azar, 435 Mass. 675, 687 (2002). Because there was an enormous amount of evidence in this case of the defendant repeatedly engaging in unlawful (and repulsive) sexual behavior while in prison that, despite its criminality, did not amount to a “sexual offense” within the meaning of the statute during the relevant time, the failure to define those words was significant. See Commonwealth v. Acevedo, 446 Mass. 435, 450 (2006). Further, the evidence with respect to the likelihood to commit a defined “sexual offense” was not “strong and one-sided,” Commonwealth v. Miranda, 22 Mass. App. Ct. 10, 21 (1986), because the parties presented competing, sophisticated expert testimony. The failure to explain the words “sexual offense” required the jury to speculate as to their meaning, and presents the serious risk that the jury either misunderstood the legal meaning of the words or disregarded them entirely. Although counsel may have chosen not to request a definition for strategic reasons, “there is no reasonable tactical basis for a failure to object to a mistaken and unfavorable (to the defendant) definition of an element of the crime.” Commonwealth v. Azar, supra at 689.6
In a case such as this, where the jury were not instructed completely on the elements of the Commonwealth’s required proof, “there is a substantial risk that a person has been convicted *552for a course of conduct that [does not make him a sexually dangerous person].” Commonwealth v. Amirault, 424 Mass. 618, 647 n.21 (1997). This risk was exacerbated by the testimony at trial. Although three of the four experts offered their opinions that exhibitionism alone could not support a finding of sexual dangerousness, these opinions did not state that this was because of a limitation in the scope of the statute. Further, these opinions are not instructions. They cannot substitute for instructions. It is the judge, not experts, who instructs the jury as to matters of law. Neither of the Commonwealth’s witnesses testified as to what future behaviors, as defined by the Legislature, could be anticipated, let alone with any specific reasonable probability. The record lacks any testimony as to which sexual offenses the defendant was likely to commit. The only relevant testimony was the bare opinion by both of the Commonwealth’s experts that the defendant had a personality disorder that made him likely to commit sexual offenses. The jurors were left to guess as to what future behaviors might constitute “sexual offenses” so as to commit the defendant as a sexually dangerous person.7
I therefore conclude that the insufficient instruction resulted in a substantial risk of a miscarriage of justice, and I would remand the case for a new trial. On remand, I should note, the jury could not be instructed by providing them with the entire list of “sexual offenses” defined by the statute, any more than a jury deciding a case of breaking and entering with intent to commit a felony may be provided with a list of all felonies. Indeed, given the powerful reactions that people may reasonably have to many of the offenses described, this would be unfairly prejudicial. Rather, the law, as noted, requires that any instruction be based upon the evidence as developed at trial, compare Commonwealth v. Gonzalez, 67 Mass. App. Ct. 877, 881 (2006) (judge not required to charge the jury on a specific theory of defense if there is no rational basis for it in the evidence), and should be the product of active conversation between the judge and counsel. Cf. 6A Wash. Prac., Wash. Pattern Jury Instr. Civ. WPI 365.16 (5th ed. 2005) (for similar statute, suggesting that analogue to “sexual offense” be defined with reference to the specific *553crimes the evidence suggests the defendant is “likely to commit in the future”). I say this mindful of the trial judge’s primary role in formulating legal instructions specifically applicable to the particular issues and evidence in a case, as experienced during trial.
The failure to instruct the words “sexual offense” to the jury left them to their uninstructed imaginations in deciding this case, and thus created a substantial risk of a miscarriage of justice. I would reverse the judgment and remand for further proceedings.
The judge instructed, in relevant part: “Our Legislature has defined the sexually dangerous person as any person who has been convicted of a sexual offense and who suffers from a mental abnormality or personality disorder which makes the person likely to engage in sexual offenses if not confined to a secure facility. . . . [T]he Commonwealth must prove each of three elements beyond a reasonable doubt: first, that the defendant has been convicted of a sexual offense; second, that he suffers from a personality disorder, and third, that as a result of the personality disorder, he is likely to engage in sexual offenses if not confined to a secure facility.”
General Laws c. 272, § 16, as amended by St. 1987, c. 43, makes unlawful “open and gross lewdness and lascivious behavior.”
The current version of G. L. c. 123A, § 1, was amended by St. 2004, c. 66, §§ 1-6, and St. 2008, c. 451, § 84.
The court contends that the jury should be permitted to apply their own common sense as to the definition of a “sexual offense” within the meaning of the statute. I disagree. While respectful of the role that common sense can play in our system of justice, I conclude that the possible meanings of “sexual offense,” like the possible meanings of “felony,” both technical terms defined by statute, cannot be left to the jury’s uninstructed imaginations.
I agree with the majority that the substantial risk analysis is appropriate in this case.
I have considered the possibility that the defendant’s trial counsel chose not to seek an instruction defining “sexual offense” as a deliberate litigation tactic. It would certainly have been a legitimate goal of the attorney to avoid having the judge read the litany of offenses in the Legislature’s definition. My reading of the record leads me to conclude that this issue was ignored by both the attorneys and the judge, and that strategic evaluation by the attorney played no role in the inattention. Furthermore, I would be hard-pressed to accept the result here, where the attorney made no effort to ask the judge to design a definitional instruction based upon the Commonwealth’s evidence and where the judge bears the responsibility, as previously noted, to design an instruction limited to the legitimate issues in the case and based upon the evidence presented.
One might ask, “[J]ust what sexual offense do you imagine the jurors unanimously utilized in reaching their verdict?”