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]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "birthday":
   anniversary, annual holiday, bicentenary, bicentennial, biennial,
   bissextile day, centenary, centennial, commemoration, decennial,
   diamond jubilee, golden wedding anniversary, holy days,
   immovable feast, jubilee, leap year, name day, natal day,
   octennial, quadrennial, quasquicentennial, quincentenary,
   quincentennial, quinquennial, septennial, sesquicentennial,
   sextennial, silver wedding anniversary, tercentenary,
   tercentennial, tricennial, triennial, wedding anniversary



Easton's 1897 Bible Dictionary:

Birth-day
   The observance of birth-days was common in early times (Job 1:4,
   13, 18). They were specially celebrated in the land of Egypt
   (Gen. 40:20). There is no recorded instance in Scripture of the
   celebration of birth-days among the Jews. On the occasion of
   Herod's birth-day John the Baptist was beheaded (Matt. 14:6).