Longman Dictionary of Contemporary English - apostrophe

~ n a) the sign (') used in writing to show that numbers or letters have been left out, as in 'don't' (=do not) and '86 (=1986) b) the same sign used before 's' to show that something belongs to someone or something, or is connected with them, as in 'John's book', or 'Charles' mother', or 'Nixon's last year as president' c) used before 's' to show the plural of letters and numbers as in 'Your r's look like v's.'