What is mental? What is mental? Of or relating to the mind; intellectual: mental powers. Executed or performed by the mind; existing in the mind: mental images of happy times. Of, relating to, or affected by a disorder of the mind. Intended for treatment of people affected with disorders of the mind.Of or relating to telepathy or mind reading.Slang Emotionally upset; crazed: got mental when he saw the dent in his new car. Offensive Slang Slang Mentally or psychologically disturbed.Of or relating to the chin.