The Collaborative International Dictionary of English v.0.48:

Tether \Teth"er\ (t[ect][th]"[~e]r), n. [Formerly tedder, OE.
   tedir; akin to LG. tider, tier, Icel. tj[=o][eth]r, Sw.
   tj[=u]der, Dan. t["o]ir. [root]64.]
   A long rope or chain by which an animal is fastened, as to a
   stake, so that it can range or feed only within certain
   limits.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tether \Teth"er\, v. t. [imp. & p. p. Tethered; p. pr. & vb.
   n. Tethering.]
   To confine, as an animal, with a long rope or chain, as for
   feeding within certain limits.
   [1913 Webster]

         And by a slender cord was tethered to a stone.
                                                  --Wordsworth.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tether
    n 1: restraint consisting of a rope (or light chain) used to
         restrain an animal [syn: leash, tether, lead]
    v 1: tie with a tether; "tether horses"