Source : Webster's Revised Unabridged Dictionary (1913)

Apostrophe \A*pos"tro*phe\, n. [(1) L., fr. Gr. ? a turning
   away, fr. ? to turn away; ? from + ? to turn. (2) F., fr. L.
   apostrophus apostrophe, the turning away or omitting of a
   letter, Gr. ?.]
   1. (Rhet.) A figure of speech by which the orator or writer
      suddenly breaks off from the previous method of his
      discourse, and addresses, in the second person, some
      person or thing, absent or present; as, Milton's
      apostrophe to Light at the beginning of the third book of
      ``Paradise Lost.''

   2. (Gram.) The contraction of a word by the omission of a
      letter or letters, which omission is marked by the
      character ['] placed where the letter or letters would
      have been; as, call'd for called.

   3. The mark ['] used to denote that a word is contracted (as
      in ne'er for never, can't for can not), and as a sign of
      the possessive, singular and plural; as, a boy's hat,
      boys' hats. In the latter use it originally marked the
      omission of the letter e.

   Note: The apostrophe is used to mark the plural of figures
         and letters; as, two 10's and three a's. It is also
         employed to mark the close of a quotation.

Source : WordNet®

apostrophe
     n 1: address to an absent or imaginary person
     2: the mark (') used to indicate the omission of one or more
        letters from a printed word

Source : Free On-Line Dictionary of Computing

apostrophe
     
        {single quote}