What is the legal definition of a crime?

"Q. In other words, if I have a gun on you right now and demand your money, what is that?

"A. It's a robbery." (People v. Torres, supra, 33 Cal.App.4th at p. 44.)

The appellate court held that this testimony was inadmissible, for three reasons. First, the expert improperly gave an opinion on the legal definition of a crime. (People v. Torres, supra, 33 Cal.App.4th at pp. 45-46.) Second, the expert improperly gave an opinion concerning the defendant's guilt. (Id . at pp. 46-47.) The court explained: "A consistent line of authority in California as well as other jurisdictions holds a witness cannot express an opinion concerning the guilt or innocence of the defendant. [Citations.] . . . [T]he reason . . . is not because guilt is the `ultimate issue of fact' to be decided by the jury. Opinion testimony often goes to the ultimate issue in the case. [Citations.] Rather, opinions on guilt or innocence are inadmissible because they are of no assistance to the trier of fact. To put it another way, the trier of fact is as competent as the witness to weigh the evidence and draw a conclusion on the issue of guilt." (Id. at pp. 46-47.) Third, the expert improperly gave an opinion as to whether a crime had been committed. The court reasoned by analogy to the rule that an expert cannot give an opinion that the defendant is guilty. (Id. at pp. 47-48.)