The Collaborative International Dictionary of English v.0.48:

Birthmark \Birth"mark`\, n.
   Some peculiar mark or blemish on the body at birth.
   [1913 Webster]

         Most part of this noble lineage carried upon their body
         for a natural birthmark, . . . a snake.  --Sir T.
                                                  North.
   [1913 Webster]

WordNet (r) 3.0 (2006):

birthmark
    n 1: a blemish on the skin that is formed before birth [syn:
         birthmark, nevus]