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):

tightening
    n 1: the act of making something tighter; "the tightening of
         economic controls" [ant: laxation, loosening]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "tightening":
   accelerando, acceleration, aggravation, beefing-up, blowing up,
   blowup, clamping, clamping down, compression, concentration,
   condensation, consolidation, crescendoing, crescent, crush,
   deepening, enhancement, exacerbation, exaggeration, expanding,
   explosion, growing, heating-up, heightening, increasing,
   incremental, information explosion, intensification, intensifying,
   lengthening, magnification, multiplying, nip, on the increase,
   pickup, pinch, population explosion, press, pressure,
   proliferating, redoubling, reinforcement, snowballing, speedup,
   spreading, squeeze, squeezing, step-up, strengthening, swelling,
   tweak, waxing