The Collaborative International Dictionary of English v.0.48:

Tighten \Tight"en\, v. t. [imp. & p. p. Tightened; p. pr. &
   vb. n. Tightening.]
   To draw tighter; to straiten; to make more close in any
   manner.
   [1913 Webster]

         Just where I please, with tightened rein
         I'll urge thee round the dusty plain.    --Fawkes.
   [1913 Webster]

   Tightening pulley (Mach.), a pulley which rests, or is
      forced, against a driving belt to tighten it.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tighten
    v 1: make tight or tighter; "Tighten the wire" [syn: tighten,
         fasten]
    2: become tight or tighter; "The rope tightened"
    3: restrict; "Tighten the rules"; "stiffen the regulations"
       [syn: stiffen, tighten, tighten up, constrain]
    4: narrow or limit; "reduce the influx of foreigners" [syn:
       reduce, tighten]

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "tighten":
   affix, anchor, annex, attach, back, belay, box, box in, box up,
   brace, cement, cinch, circumscribe, clamp, clinch, close, compress,
   confine, continue, cramp, crank, crank in, crush, drag out, draw,
   draw in, draw out, draw taut, elongate, engraft, ensphere, extend,
   fasten, fix, graft, grapple, hem, hem in, keep from spreading,
   keep within bounds, knit, lengthen, lengthen out, let out, limit,
   localize, make fast, moor, narrow, nip, pinch, press, produce,
   prolong, prolongate, protract, pull, pull in, put to, qualify,
   reel, reel in, reinforce, restrict, rigidify, screw up, secure,
   set, set to, shore up, spin out, squeeze, stiffen, stint, strain,
   strengthen, stretch, stretch out, string out, tackle, tauten,
   tense, trice up, trim, tweak, wad up, winch, wind in, windlass