axillary. adj. 1. (Anatomy) of, relating to, or near the armpit. 2. (Botany) botany growing in or related to the axil: an axillary bud. n, pl-laries. 3. (Zoology) Also called: axillar (usually plural) one of the feathers growing from the axilla of a bird's wing. (ˈaek ˈæk səˌlɛr). i., adj., n. pl