/ əˈpɒstrəfi; NAmE əˈpɑːs-/ noun
1.
the mark ( ' ) used to show that one or more letters or numbers have been left out, as in she's for she is and '63 for 1963
2.
the mark ( ' ) used before or after the letter 's' to show that sth belongs to sb, as in Sam's watch and the horses' tails
3.
the mark ( ' ) used before the letter 's' to show the plural of a letter or number, as in How many 3's are there in 9? and There are two m's in 'comma'.
••
WORD ORIGIN
mid 16th cent. (denoting the omission of one or more letters): via late Latin , from Greek apostrophos accent of elision (showing a certain part of a word to be left out when pronouncing it), from apostrephein turn away, from apo from + strephein to turn.