apostrophe (n.) Look up apostrophe at Dictionary.com
mark indicating omitted letter, 1580s, from Middle French apostrophe, from Late Latin apostrophus, from Greek apostrophos (prosoidia) "(the accent of) turning away," thus, a mark showing where a letter has been omitted, from apostrephein "avert, turn away," from apo "off, away from" (see apo-) + strephein "to turn" (see strophe).

In English, the mark often represents loss of -e- in -es, possessive ending. It was being extended to all possessives, whether they ever had an -e- or not, by 18c. Greek also used this word for a "turning aside" of an orator in speech to address some individual, a sense first recorded in English 1530s.