compos mentis
adjective·/ˌkɑːm.poʊs ˈmɛn.tɪs/
Of sound mind; in possession of one's mental faculties, especially in a legal or formal judgment of capacity (formal). Compos mentis is measured and evidentiary, a mind accounted for.
The will was upheld because, on the day she signed it, she was compos mentis—clear-eyed, steady, and uncoerced.
Etymology
From Latin compos mentis "in control of the mind," literally "master of one's mind," from compos "having control" + mentis, genitive of mens "mind." English preserves the phrase as a small island of Latin in modern law.
Related Words
capacitythe legal concept often at issue
lucidclear-minded; less formal
non compos mentisthe traditional opposite phrase
competenteveryday legal-adjacent synonym