's

from The American Heritage® Dictionary of the English Language, 5th Edition.

• Is.
• Has.
• Does.
• Us.

from the GNU version of the Collaborative International Dictionary of English.

• possessive noun A contraction for is or (colloquially) for has.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb contracted form of is
• verb contracted form of has
• verb informal contracted form of does (used only with the auxiliary meaning of does and only after interrogative words)
• verb nonstandard are
• pronoun 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.
• pronoun nonstandard Contracted form of as in its nonstandard use as a relative pronoun.
• suffix 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.
• suffix proscribed Used to form the plural of nouns that correctly take just an "s" in the plural. See greengrocer’s apostrophe.
• preposition Possessive marker, indicating than an object belongs to the noun phrase bearing the marker.
• preposition In the absence of a specified object, used to indicate “the house/place/establishment of”.
• suffix Indicates a purpose or a user.