| Noun | 1. | axillary fossa - the hollow under the arm where it is joined to the shoulder; "they were up to their armpits in water"arteria axillaris, axillary artery - the part of the main artery of the arm that lies in the armpit and is continuous with the subclavian artery above and the brachial artery below shoulder - the part of the body between the neck and the upper arm | |