A respirator being held over Joe Tormolen's mouth

A respirator was a medical device which assisted a patient who was having trouble breathing.

While undergoing surgery to repair his self-inflicted wounds, Joe Tormolen was on a respirator. (TOS: "The Naked Time")

Leonard McCoy considered putting Spock on a respirator when he was in the final stages of choriocytosis. (TAS: "The Pirates of Orion")