Source : Webster's Revised Unabridged Dictionary (1913)

Third \Third\ (th[~e]rd), a. [OE. thirde, AS. [thorn]ridda, fr.
   [thorn]r[=i], [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[=i]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.

   {Third estate}.
      (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},
      n., 7.

   {Third sound}. (Mus.) See {Third}, n., 3.

Source : WordNet®

third estate
     n : the third estate of the realm; the commons (especially in
         Britain or France) viewed as forming a political order
         having representation in a parliament