A star year was a unit of time.
As of 2269, recent joint discussions between the Federation and the Klingon and Romulan Empires had revealed that a starship had disappeared in a certain sector precisely every 27.346 star years. (TAS: "The Lorelei Signal")
Kirk also implied that "Earth Federation" ships had been disappearing in the sector for 150 (regular) years, perhaps offering some clue as to the length of star years,