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old unit of measure equal to one-third of a pipe (42 gallons), 1530s, from Anglo-French ters, Old French tierce, from Latin tertia, fem. of tertius "a third," from root of tres "three" (see three). Also used in Middle English for "a third part" (late 15c.), "the third hour of the canonical day" (ending at 9 a.m.), late 14c., and, in astronomy and geometry, "sixtieth part of a second of an arc."