From WordNet (r) 2.0 (August 2003) [wn]:
axilla
n : the hollow under the arm where it is joined to the shoulder;
"they were up to their armpits in water" [syn: armpit,
axillary cavity, axillary fossa]
From The Collaborative International Dictionary of English v.0.48 [gcide]: Axilla \Ax*il"la\ ([a^]ks*[i^]l"l[.a]), n.; pl. Axillae ([a^]ks*[i^]l"l[=ae]). [L.] (Anat.) The armpit, or the cavity beneath the junction of the arm and shoulder. [1913 Webster] 2. (Bot.) An axil. [1913 Webster]