A contraction is a word or phrase that has been shortened by dropping one or more letters.
In writing, an apostrophe takes the place of the missing letters.
In writing, an apostrophe takes the place of the missing letters.
http://english-zone.com/spelling/cont-01.html
http://a4esl.org/q/f/z/zz45fck.htm
http://www.quia.com/hm/44478.html