The most demanding Vulcan mental discipline, a years-long course of study under the masters of Gol intended to shed every last trace of emotion in favour of pure logic.
Kolinahr is the Vulcan ritual and discipline by which an adept seeks to purge all remaining emotion and live by total logic. It is taught by the masters on the plateau of Gol and takes years to complete, ending in a ceremony in which the student receives a symbol of achievement.
In Star Trek: The Motion Picture, Spock has left Starfleet to undertake Kolinahr. At the final ceremony the presiding master senses that the consciousness of V'Ger has stirred his human half, and he leaves Vulcan without completing it.
Voyager establishes that Tuvok also studied the discipline during his years away from Starfleet, a period recalled in Flashback, and abandoned it when pon farr led him to marry T'Pel. Few Vulcans ever attempt it, and fewer finish.