(covers information from several alternate timelines)
Fainting, to pass out or to experience unconsciousness was the sudden loss of consciousness, often accompanied by a fall. One could be said to be knocked out if the loss of consciousness was due to the application of force, such as a punch or striking one's head on something.
During his brief visit to the mirror universe in 2267, Dr. Leonard McCoy was appalled by what he found, calling his sickbay a "Chamber of Horrors", further describing that his counterpart's assistants "were betting on the tolerance of an injured man," specifically, "how long it would take him to pass out from the pain." (TOS: "Mirror, Mirror")
Klingons generally didn't faint, except when ill. In 2365, after Worf had fainted on the bridge of the USS Enterprise-D, Doctor Katherine Pulaski described the cause of fainting as suffering a dramatic drop in blood pressure, blood glucose level dropping, and having deficient blood flow resulting from circulatory failure. Despite his protests, she checked the computer and diagnosed him with rop'ngor. (TNG: "Up The Long Ladder")