Real Dictionary

apostrophe
countable noun: An apostrophe is the mark ' when it is written to indicate that one or more letters have been left out of a word, as in 'isn't' and 'we'll'. It is also added to nouns to form possessives, as in 'Mike's car'.

apostrophe in American English 1
words addressed to a person or thing, whether absent or present, generally in an exclamatory digression in a speech or literary writing
noun: the sign ('), as used: to indicate the omission of one or more letters in a word, whether unpronounced, as in o'er for over, or pronounced, as in gov't for government; to indicate the possessive case, as in man's; or to indicate plurals of abbreviations a

apostrophe in American English 2
a mark (') used:
noun: a digression in the form of an address to someone not present, or to a personified object or idea, as “O Death, where is thy sting?”

apostrophe in British English 1
noun: the punctuation mark ' used to indicate the omission of a letter or number, such as he's for he has or he is, also used in English to form the possessive, as in John's father and twenty pounds' worth

apostrophe in British English 2
noun: a digression from a discourse, esp an address to an imaginary or absent person or a personification