The Collaborative International Dictionary of English v.0.48:

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."
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
         [1913 Webster]

WordNet (r) 3.0 (2006):

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

The Free On-line Dictionary of Computing (30 December 2018):

single quote
'
apostrophe

    "'" ASCII character 39.

   Common names include single quote; quote; ITU-T: apostrophe.
   Rare: prime; glitch; tick; irk; pop; INTERCAL: spark;
   ITU-T: closing single quotation mark; ITU-T: acute accent.

   Single quote is used in C and derived languages to introduce
   a single character literal value which is represented
   internally by its ASCII code.  In the Unix shells and
   Perl single quote is used to delimit strings in which
   variable substitution is not performed (in contrast to
   double-quote-delimited strings).

   Single quote is often used in text for both open and close
   single quotation mark and apostrophe.  Typesetters use two
   different symbols - open has a tail going up, close and
   apostrophe have tails hanging down (like a raised comma).
   Some people use back quote (`) for open single quotation
   mark.

   (1998-04-04)