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]