: a mark ' used to indicate the omission of letters or figures, the possessive case (as in "John's book"), or the plural of letters or figures (as in "the 1960's")
In the contraction "can't," the apostrophe replaces two of the letters in the word "cannot.".
: the addressing of a usually absent person or a usually personified thing rhetorically
Carlyle's "O Liberty, what things are done in thy name!" is an example of apostrophe.
Word History
Etymology
Noun (1)
borrowed from French & Late Latin; French, borrowed from Late Latin apostrophus, apostrophos "mark placed above a consonant to indicate that a following vowel has been deleted," borrowed from Greek apóstrophos (feminine noun, presumably shortened from the collocation apóstrophos prosōidía, with prosōidía in sense "accent mark"), from apóstrophos, adjective, "turned away, averted," derivative of apostréphein "to turn back, turn away" — more at apostrophe entry 2
Note: The sources of English apostrophe imply that the word would have been pronounced with three syllables, but pronunciation with four syllables, copying apostrophe entry 2, was general by at least the time of the Oxford English Dictionary, first edition (1885). An early occurrence in Shakespeare's Love's Labor Lost, 1598 ("You finde not the apostraphas, and so misse the accent") is apparently directly from Latin. The motivation for the sense "turned away, averted" is uncertain. Classical scholia explain apóstrophos variously as referring to the bent shape of the mark, or to its function as averting hiatus (see W.S. Allen, Vox Graeca, second edition, Cambridge, 1974, p. 94; according to Allen, "the latter explanation seems the more probable").
Noun (2)
borrowed from Latin apostropha, borrowed from Greek apostrophḗ "turning back or away, (in rhetoric) turning away from a group of hearers to a single person," noun derivative of apostréphein "to turn back, turn away, avert," from apo-apo- + stréphein "to turn, twist" — more at strophe