The Collaborative International Dictionary of English v.0.48:
Third \Third\ (th[~e]rd), a. [OE. thirde, AS. [thorn]ridda, fr.
[thorn]r[imac], [thorn]re['o], three; akin to D. derde third,
G. dritte, Icel. [thorn]ri[eth]i, Goth. [thorn]ridja, L.
tertius, Gr. tri`tos, Skr. t[.r]t[imac]ya. See Three, and
cf. Riding a jurisdiction, Tierce.]
1. Next after the second; coming after two others; -- the
ordinal of three; as, the third hour in the day. "The
third night." --Chaucer.
2. Constituting or being one of three equal parts into which
anything is divided; as, the third part of a day.
(a) In England, the commons, or the commonalty, who are
represented in Parliament by the House of Commons.
(b) In France, the tiers ['e]tat. See Tiers ['e]tat.
Third order (R. C. Ch.), an order attached to a monastic
order, and comprising men and women devoted to a rule of
pious living, called the third rule, by a simple vow if
they remain seculars, and by more solemn vows if they
become regulars. See Tertiary, n., 1.
Third person (Gram.), the person spoken of. See Person,
Third sound. (Mus.) See Third, n., 3.