Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

   {Tightening pulley} (Mach.), a pulley which rests, or is
      forced, against a driving belt to tighten it.

Source : WordNet®

tightening
     n : the act of making something tighter; "the tightening of
         economic controls" [ant: {loosening}]