Pronunciation: (&thslash;âr; unstressed &thslash;ur),[key] —pron. 1. a form of the possessive case of they used as an attributive adjective, before a noun: their home; their rights as citizens; their departure for Rome. 2. (used after an indefinite singular antecedent in place of the definite masculine form his or the definite feminine form her): Someone left their book on the table. Did everyone bring their lunch? Cf. theirs.