Other forms: triennially; triennials
Use triennial to describe anything that happens every three years. Your family just held its triennial reunion, so you're going to have to wait three years to see those distant cousins again.
Meetings, conferences, and art shows are some of the many things that might occur every three years, or on a triennial basis. In Judaism, the triennial cycle is an ancient practice of reading the entire Torah over the course of a three-year period. Triennial can also mean "lasting for three years," like a sports player's triennial, or three-year, contract. The word derives from Latin roots meaning "three" and "yearly."