The Collaborative International Dictionary of English v.0.48:

Birthday \Birth"day`\ (b[~e]rth"d[=a]`), n.
   1. The day in which any person is born; day of origin or
      commencement.
      [1913 Webster]

            Those barbarous ages past, succeeded next
            The birthday of invention.            --Cowper.
      [1913 Webster]

   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.
      [1913 Webster]

            This is my birthday; as this very day
            Was Cassius born.                     --Shak.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Birthday \Birth"day`\, a.
   Of or pertaining to the day of birth, or its anniversary; as,
   birthday gifts or festivities.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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: birthday, natal
       day]