Real Dictionary

auxin in American English
a plant hormone, esp. indoleacetic acid, produced in the fruits, seeds, leaves, or stem tips, to promote longitudinal growth and to control bud growth, root formation, leaf abscission, etc.: also produced synthetically
noun: a class of substances that in minute amounts regulate or modify the growth of plants, esp. root formation, bud growth, and fruit and leaf drop

auxin in British English
noun: any of various plant hormones, such as indoleacetic acid, that promote growth and control fruit and flower development. Synthetic auxins are widely used in agriculture and horticulture