The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

axilla
    n 1: the hollow under the arm where it is joined to the
         shoulder; "they were up to their armpits in water" [syn:
         armpit, axilla, axillary cavity, axillary fossa]