A Nervous Breakdown, or Mental Breakdown, is a term used by humans to describe a person suffering from an acute attack of mental illness, such as depression or anxiety. The term nervous breakdown is generally not used by mental health professionals as they feel it is not descriptive enough of either the symptoms or actual disorder a person is experiencing. Often such professionals will describe the event as a brief reactive psychosis.
Seeing Doctor Richard Daystrom in an extremely agitated state after his M-5 Computer malfunctioned and initiated deadly force against other Starfleet ships, Dr. Leonard McCoy became concerned that Daystrom was about to suffer a nervous breakdown. Kirk confirmed this by confronting Daystrom, who had to be immobilized by a nerve pinch administered by Spock. Daystrom was taken from the bridge, sedated, and confined to sickbay. Afterwards, he was hospitalized for treatment. (TOS episode: "The Ultimate Computer")