The Collaborative International Dictionary of English v.0.48:
auxin \aux"in\ n. 1. a substance which, in small concentrations, promotes root formation, bud growth, or certain other processes such as fruit ripening or leaf drop in plants. [WordNet 1.5 +PJC]WordNet (r) 3.0 (2006):
auxin n 1: a plant hormone that promotes root formation and bud growth