mentally ill

Suffering from a mental illness.

People who are mentally ill considered as a group. Often used with the.

adjective

Afflicted with or exhibiting irrationality and mental unsoundness: brainsick, crazy, daft, demented, disordered, distraught, dotty, insane, lunatic, mad, maniac, maniacal, moonstruck, off, touched, unbalanced, unsound, wrong. Informal bonkers, cracked, daffy, gaga, loony. Slang bananas, batty, buggy, cuckoo, fruity, loco, nuts, nutty, screwy, wacky. Chiefly British crackers. Law non compos mentis. Idioms: around the bend, crazy as a loon, mad as a hatter, not all there, nutty as a fruitcake, offout ofone's head, off one's rocker, of unsound mind, out of one's mind, sick in the head, stark raving mad. See sane/insane.

The adjective has one meaning:

Meaning #1: suffering from severe mental illness
  Synonyms: unsound, unstable