A respirator was a medical device which assisted a patient who was having trouble breathing during medical treatment or due to medical distress.
While undergoing surgery to repair his self-inflicted wounds in 2266, Joe Tormolen was on a respirator. (TOS: "The Naked Time")
In 2270 Leonard McCoy considered putting Spock on a respirator when the Vulcan was in the final stages of choriocytosis. (TAS: "The Pirates of Orion")