Tierce (t or ter), also known as third, is a unit of time equal to 160 of a second, 16.6 milliseconds, or 60 quatremes. Tierce comes from the Middle English word terce, tierce, from Anglo-French word terz, from Latin tertius, all meaning "third". Tierce can be used as an interval of second, such as the more precise timing of reaching a finish line in the race. In astronomy, arctierce can be used as an interval of arcsecond in addition to milliarcsecond, which is symbolized in t or ’’’ (triple prime).

A blink of an eye lasts 6–24 tierces.