A blackout was a semi-conscious state during which an individual was still able to speak and act, yet had no memory later of doing so.

In 2267, Montgomery Scott denied having blacked out when Sybo was murdered on Argelius II during his therapeutic shore leave. Instead, he claimed to have tried to come to her aid, only to be stopped by an invisible presence. His statement was proven true by a verifier scan. It came after Kirk asked the USS Enterprise's computer whether the head injury suffered by Scott which necessitated his shore leave was sufficient to cause periods of functional amnesia, and was told they were not. Regardless, Scott denied any memory of the murders of Kara and Lieutenant Karen Tracy. (TOS: "Wolf in the Fold")

In 2269, when impersonated by a Vendorian, James T. Kirk couldn't recall his supposed orders to head through the Neutral Zone. Fearing that he'd become subject to blackouts, he decided to head down to sickbay for a full medical examination. The Vendorian, now posing as Doctor Leonard McCoy, told him that he (McCoy) would do it later, as he was too busy. (TAS: "The Survivor")