Reus in classical Latin was a noun meaning one of the parties to a law-suit. In medieval Latin it was an adjective meaning guilty, and that is how it is used currently. Two important elements of a crime are the actus reus, a criminal act, and the mens rea, a criminal intention. If you walk out of the bank with a thousand euro stuck to the sole of your shoe, you probably haven’t committed a theft. Not unless you stuck it there yourself.
Mens means mind, incidentally, and is at the root of words like mental and dementia. The adjective appears in two different forms, reus and rea, because actus is a masculine noun and mens is feminine. Reum is the neuter form, and like all neuter forms it can do duty as a noun, simply meaning a guilty thing, or a crime.
You might see the full sentence actus reus non facit reum nisi mens sit rea: A guilty act does not make a man guilty (reum here is the masculine accusative) unless the mind is guilty. I think it’s a good idea to keep a phrasebook of model sentences like this, partly because they encapsulate some interesting grammar points, but partly to prepare for the glorious day when you accidentally make off with a lawyer’s breakfast. As she follows you out of the diner, cry: “Actus reus non facit reum nisi mens sit rea!”
It could be the beginning of a beautiful friendship.