The Collaborative International Dictionary of English v.0.48:

gibberellin \gibberellin\ n. (Chem.),
   Any of a number plant growth hormones, the first of which was
   isolated in 1938 from the fungus Gibberella fujikuroi; more
   than 60 related gibberelins are known. The most important is
   gibberellin A3, also called gibberellic acid. They are
   used in agriculture for promoting plant growth.
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

gibberellin
    n 1: a plant hormone isolated from a fungus; used in promoting
         plant growth