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"

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "tether":
   Oregon boat, anchor, batten, batten down, belay, bilbo, bind, bond,
   bonds, bridle, camisole, chain, chains, collar, cord, cuffs,
   enchain, entrammel, fasten, fasten down, fetter, gag, gyve, gyves,
   halter, hamper, handcuff, handcuffs, harness, hitch up, hobble,
   hobbles, hog-tie, hook up, hopple, hopples, irons, lash, lead,
   leading strings, leash, make fast, make secure, make sure, manacle,
   moor, muzzle, peg down, picket, pillory, pin down, pinion,
   put in irons, reins, restrain, restraint, restraints, rope, saddle,
   secure, shackle, stake, stocks, straightjacket, strait-waistcoat,
   straitjacket, stranglehold, strap, tie, tie down, tie up, trammel,
   trammels, yoke