Triennial describes something that happens once every three years — a triennial meeting, a triennial report, a triennial art show. It can also describe something that lasts three years, as in a triennial term of office, though the 'every three years' meaning is far more common and is what students will meet. It is a formal word, at home in official notices, company rules and academic writing; in normal speech people simply say 'every three years', which is clearer and always safe. It is also a noun: 'a triennial' is the event itself, and in the art world some big international exhibitions are called Triennials, alongside the more famous Biennials held every two years.