The Collaborative International Dictionary of English v.0.48:
one-third \one-third\ n. one of three equal parts of a divisible whole. Syn: third, tierce. [WordNet 1.5]WordNet (r) 3.0 (2006):
one-third n 1: one of three equal parts of a divisible whole; "it contains approximately a third of the minimum daily requirement" [syn: one-third, third, tierce]