What is the test for defining a crime or defense?

"As a general rule, ordinary words do not require definition; they are presumed to be understood by the jurors. [Citations.] The language of a statute defining a crime or defense is generally an appropriate and desirable basis for an instruction [citation], and is ordinarily sufficient when the defendant fails to request further amplification. [Citations.] If the jury would have no difficulty in understanding the statute without guidance, the court need do no more than instruct in statutory language. [Citation.]" (People v. Jones (1971) 19 Cal.App.3d 437, 447, 96 Cal.Rptr. 795.)