men·tal / ˈmentl/ • adj. 1. of or relating to the mind: mental faculties mental phenomena. ∎ carried out by or taking place in the mind: a quick mental calculation I started my mental journey.2. of, relating to, or suffering from disorders or illnesses of the mind: a mental hospital. ∎ inf. insane; crazy: every time I'm five minutes late, they go mental.DERIVATIVES: men·tal·ly adv.