Seppuku was a word in the Japanese language for the ritual suicide by disembowelment. It was sometimes referred to as Harakiri.
When Walter Takagi was facing his punishment in 1974 he had grisly visions of seppuku from the old samurai movies he had watched as a kid. (TOS novel: The Rise and Fall of Khan Noonien Singh, Volume 1)
Bridget McLellan referred to Starfleet's contingency plan of using trilithium bomb to seppuku. (VAN - Declassified novella: The Stars Look Down)
In an alternate timeline Doctor Georges Mordreaux told Spock of how a Starfleet officer had committed suicide in protest over Captain Hunter's dishonorable discharge from the fleet. Spock assumed that Sulu had killed himself, but did not think Sulu was the type to commit harakiri. Mordreaux corrected Spock, saying it was not Sulu but a Russian officer that he couldn't remember the name of, and that Sulu had joined the Free Commandoes. (TOS novel: The Entropy Effect)