's

American Heritage® Dictionary of the English Language, Fourth Edition

1. Is: She's here.
2. Has: He's arrived.
3. Does: What's he want?
4. Us: Let's go.

Wiktionary

1. v. contracted form of is
2. v. contracted form of has
3. v. informal contracted form of does (used only with the auxiliary meaning of does and only after interrogative words)
4. v. nonstandard are
5. pro. Contracted form of us found in the formula let’s used to form first-person plural imperatives. Let’s is now considered as a compound.
6. pro. nonstandard Contracted form of as in its nonstandard use as a relative pronoun.
7. prep. Possessive marker, indicating than an object belongs to the noun phrase bearing the marker.
8. prep. In the absence of a specified object, used to indicate “the house/place/establishment of”.
9. n. Indicates a purpose or a user.
10. n. usage problem Used to form the plurals of numerals, letters, some abbreviations and some nouns, usually because the omission of an apostrophe would make the meaning unclear or ambiguous.
11. n. proscribed Used to form the plural of nouns that correctly take just an "s" in the plural. See greengrocer’s apostrophe.

GNU Webster's 1913

1. A contraction for is or (colloquially) for has.