2. In law, competent to go to trial.
3. If someone is compos mentis, he/she is able to think clearly and is responsible for her/his actions: "My sister was quite old, when I last saw her, but she was definitely compos mentis."
A person who is in his/her right mind is adjudged compos mentis; literally, as "in full possession of mental powers". This term and non compos mentis are properly used in legal writing and court testimony and are not supposed to be used loosely by people "unqualified to make either judgment" of another person.
Said of a lunatic, idiot, drunkard, or one who has lost his or her memory and understanding by accident or as a result of a disease.
Non compos mentis is the legal expression used for any form of mental unsoundness. See compos mentis for the opposite condition.