Source : Webster's Revised Unabridged Dictionary (1913)

Axilla \Ax*il"la\, n.; pl. {Axillae}. [L.] (Anat.)
   The armpit, or the cavity beneath the junction of the arm and
   shoulder.

   2. (Bot.) An axil.

Source : WordNet®

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}]
     [also: {axillae} (pl)]