Definify.com
Webster 1913 Edition
Their
Their
,p
ron.
& Adj.
[OE.
thair
, fr. Icel. þeirra
, þeira
, of them, but properly gen. pl. of the definite article; akin to AS. ðāra
, ðǣra
, gen. pl. of the definite article, or fr. AS. ðǣra
, influenced by the Scandinavian use. See That
.] The possessive case of the personal pronoun they;
as,
. their
houses; their
country☞ The possessive takes the form theirs ([GREEK]) 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
’Twixt our best actions and the worst of
theirs
. Denham.
Webster 1828 Edition
Their
THEIR
, a pronom.1.
Their has the sense of a pronominal adjective, denoting of them, or the possession of two or more; as their voices; their garments; their houses; their land; their country.2.
Theirs is used as a substitute for the adjective and the noun to which it refers, and in this case, it may be the nominative to a verb. 'Our land is the most extensive, but theirs is the best cultivated.' Here theirs stands as the representative of their land, and is the nominative to is. Nothing but the name of zeal appears
'Twixt our best actions and the worst of theirs.
In this use, theirs is not in the possessive case, for then there would be a double possessive.