Source : Webster's Revised Unabridged Dictionary (1913)

Birthday \Birth"day`\ (b[~e]rth"d[=a]`), n.
   1. The day in which any person is born; day of origin or
      commencement.

            Those barbarous ages past, succeeded next The
            birthday of invention.                --Cowper.

   2. The day of the month in which a person was born, in
      whatever succeeding year it may recur; the anniversary of
      one's birth.

            This is my birthday; as this very day Was Cassius
            born.                                 --Shak.

Birthday \Birth"day`\, a.
   Of or pertaining to the day of birth, or its anniversary; as,
   birthday gifts or festivities.

Source : WordNet®

birthday
     n 1: an anniversary of the day on which a person was born (or the
          celebration of it)
     2: the date on which a person was born [syn: {natal day}]