[aux.in] n [ISV, fr. Gk auxein] (1934) 1: any of various usu. acidic organic substances that promote cell elongation in plant shoots and usu. regulate other growth processes (as root initiation): as a: indoleacetic acid b: any of various synthetic substances (as 2,4-D) resembling indoleacetic acid in activity and used esp. in research and agriculture
2: plant hormone -- aux.in.ic adj