apostrophe:
/
a punctuation mark (’) used to indicate either possession (e.g., Harry's book; boys' coats) or the omission of letters or numbers (e.g., can't; he's; class of ’99):
/
"mark indicating an omitted letter," 1580s, from 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 strepto-).

单数就是‘s，复数就是s’