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Used to form the possessive case of singular nouns, plural nouns that do not end in s, certain pronouns, and phrases that function as nouns or pronouns: nation's; women's; another's; the girl next door's cat.
[Middle English -s, -es, from Old English -es, genitive sing. suff.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
1. forming the possessive singular of nouns and some pronouns: man's; one's.
2. forming the possessive plural of nouns whose plurals do not end in -s: children's.
3. forming the plural of numbers, letters, or symbols: 20's; p's and q's.
4. informal contraction of is or has: he's here; John's coming; it's gone.
5. informal contraction of us with let: let's.
6. informal contraction of does in some questions: where's he live?; what's he do?.
[senses 1, 2: assimilated contraction from Middle English -es, from Old English, masculine and neuter genitive singular; sense 3, equivalent to -s1]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014