Pronunciation: (&thslash;ârz), [key]
1. a form of the possessive case of they used as a predicate adjective, after a noun or without a noun: Are you a friend of theirs? It is theirs.
2. (used after an indefinite singular antecedent in place of the definite masculine form his or the definite feminine form hers): I have my book; does everyone else have theirs?
3. that which belongs to them: Theirs is the largest house on the block.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.