birthday

[burth-dey]
noun
1.
the anniversary of a birth.
2.
the day of a person's birth.
3.
a day marking or commemorating the origin, founding, or beginning of something.
4.
the festivities or celebration marking such a day or anniversary.

Origin:
1350–1400; Middle English; see birth, day

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World English Dictionary
birthday (ˈbɜːθˌdeɪ)
 
n
1.  a.  an anniversary of the day of one's birth
 b.  (as modifier): birthday present
2.  the day on which a person was born
3.  any anniversary

Collins English Dictionary - Complete & Unabridged 10th Edition
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Etymonline
Word Origin & History

birthday
O.E. byrddæg, "anniversary celebration of someone's birth" (usually a king or saint at first), from birth + day. Meaning "day on which one is born" is from 1570s. Birthnight is attested from 1620s.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Consider using the date of your birthday each month to remind you to go out
  exploring.
Have each student commit to doing an exploration activity on the date of their
  birthday each month.
Perhaps they had a bad experience with an unprofessional clown at a birthday
  party.
Follow the link below to enter your name and birthday message.
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