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.
When several Enterprise crew members were under the effects of pollen on an uninhabited planet in 2151, some of them lost consciousness, while the rest became delusional and paranoid. Charles Tucker III asked T'Pol, who he thought was plotting against him, if she was "waiting for [him] to pass out". (ENT: "Strange New World").
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")
Sylvia Tilly fainted after inhaling volcanic vapor. (DIS: "Will You Take My Hand?")
In 2268, half the crew of the USS Enterprise fainted due to the effects of the space amoeba. (TOS: "The Immunity Syndrome")
In 2269, Lieutenant Mira Romaine fainted while under the influence of the Zetarians. (TOS: "The Lights of Zetar")
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")
When Clare Raymond was awoken from cryogenic stasis, she fainted in shock due to seeing Worf. (TNG: "The Neutral Zone")
In 2366, the alien posing as Bolian Mitena Haro claimed to have fainted while studying before being abducted. (TNG: "Allegiance")
When the Paxans erased the Enterprise crew's memories of them, this caused them all to lose consciousness. (TNG: "Clues")
In 2370, Quark fainted upon learning that Pel was a female. (DS9: "Rules of Acquisition")
Later that year, Admiral Erik Pressman joked that William T. Riker looked as though he were about to faint when they came face to face. (TNG: "The Pegasus")
In 2372, Rom credited fainting with having saved his life, as it necessitated his visit to Deep Space 9's infirmary. (DS9: "Bar Association")
In 2377, Tom Paris thanked Seven of Nine and Icheb for assisting a pregnant B'Elanna Torres when she had a dizzy spell, which he described as "fainting" even though she never actually lost consciousness. (VOY: "Lineage")
In the writers' first draft script of ENT: "Dear Doctor", Doctor Phlox was about to bandage a bloody laceration on Trip Tucker's hand when a large, tough-looking engineering officer standing nearby suddenly fainted at the sight of the blood.