Source : Webster's Revised Unabridged Dictionary (1913)

Valentine \Val"en*tine\, n.
   1. A sweetheart chosen on St. Valentine's Day.

   2. A letter containing professions of love, or a missive of a
      sentimental, comic, or burlesque character, sent on St.
      Valentine's Day.

   {St. Valentine's Day}, a day sacred to St. Valentine; the
      14th of February. It was a very old notion, alluded to by
      Shakespeare, that on this day birds begin to mate. Hence,
      perhaps, arose the custom of sending love tokens at that
      time.

Source : WordNet®

valentine
     n 1: a sweetheart chosen to receive a greeting on Saint
          Valentine's Day; "will you be my valentine?"
     2: a card sent or given (as to a sweetheart) on Saint
        Valentine's Day