armhole

from WordNet (r) 3.0 (2006)

armhole
    n 1: a hole through which you put your arm and where a sleeve
         can be attached

from The Collaborative International Dictionary of English v.0.48

Armhole \Arm"hole`\, n. [Arm + hole.]
   1. The cavity under the shoulder; the armpit. --Bacon.
      [1913 Webster]

   2. A hole for the arm in a garment.
      [1913 Webster]