The Collaborative International Dictionary of English v.0.48:

Slow \Slow\, v. t. [imp. & p. p. Slowed; p. pr. & vb. n.
   Slowing.]
   To render slow; to slacken the speed of; to retard; to delay;
   as, to slow a steamer. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

slowing
    n 1: a decrease in rate of change; "the deceleration of the arms
         race" [syn: deceleration, slowing, retardation] [ant:
         acceleration]