What is Actus Non Facit Reum, Nisi Mens Sit Rea?

Legal Definition
An act does not make [the doer of it] guilty, unless the mind be guilty; that is, unless the intention be criminal. 3 Inst. 107. The intent and the act must both concur to constitute the crime. Lord Kenyon, C. J., 7 Term 514; Bcoom, Max. 306.
-- Black's Law Dictionary
Legal Definition
No act is a crime unless there be criminal intent. See 25 L. R. A. (N. S.) 661.
-- Ballentine's Law Dictionary