\Their\, pron. & a. [OE. thair, fr. Icel. [thorn]eirra,
[thorn]eira, of them, but properly gen. pl. of the definite
article; akin to AS. [eth][=a]ra, [eth][=ae]ra, gen. pl. of
the definite article, or fr. AS. [eth][=ae]ra, influenced by
the Scandinavian use. See {That}.]
The possessive case of the personal pronoun they; as, their
houses; their country.
Note: The possessive takes the form theirs (?) when the noun
      to which it refers is not expressed, but implied or
      understood; as, our land is richest, but theirs is best
      cultivated.
            Nothing but the name of zeal appears 'Twixt our
            best actions and the worst of theirs. --Denham.