Massachusetts Criminal Practice Jury Instruction (MCPJI)
MA CPJI §1.1. Reasonable Doubt.
The burden is on the Commonwealth to prove beyond a reasonable doubt that the defendant is guilty of the charge(s) made against him (her).
Then what is reasonable doubt? It is a term often used, probably pretty well understood, but not easily defined. It is not mere possible doubt, because every thing relating to human affairs, and depending on [fallible] evidence, is open to some possible or imaginary doubt. It is that state of the case, which, after the entire comparison and consideration of all the evidence, leaves the minds of jurors in that condition that they cannot say they feel an abiding conviction, to a moral certainty, of the truth of the charge.
The burden of proof [persuasion] is on the prosecutor. All the presumptions of law independent of evidence are in favor of innocence; and every person is presumed to be innocent until he is proved guilty. If upon such proof there is reasonable doubt remaining, the accused is entitled to the benefit of it by an acquittal.
For it is not sufficient to establish a probability, though a strong one arising from the doctrine of chances, that the fact charged is more likely to be true than the contrary; but the evidence must establish the truth of the fact to a reasonable and moral certainty; a certainty that convinces and directs the understanding and satisfies the reason and judgment, of those who are bound to act conscientiously upon it. This we take to be proof beyond reasonable doubt.