The Collaborative International Dictionary of English v.0.48:

Widen \Wid"en\, v. t. [imp. & p. p. Widened; p. pr. & vb. n.
   Widening.]
   To make wide or wider; to extend in breadth; to increase the
   width of; as, to widen a field; to widen a breach; to widen a
   stocking.
   [1913 Webster]

WordNet (r) 3.0 (2006):

widening
    n 1: an increase in width [syn: widening, broadening] [ant:
         narrowing]
    2: a part of a road that has been widened to allow cars to pass
       or park [syn: turnout, widening]
    3: the act of making something wider [syn: widening,
       broadening] [ant: narrowing]