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]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "widening":
   access, accession, accretion, accrual, accruement, accumulation,
   addition, adjunct, advance, aggrandizement, ampliation,
   amplification, appreciation, ascent, augmentation, ballooning,
   bloating, boom, boost, broadening, buildup, crescendo, deployment,
   development, dispersion, edema, elevation, enlargement, expansion,
   extension, fanning out, flare, flood, gain, greatening, growth,
   gush, hike, hiking, increase, increment, inflation, jump, leap,
   magnification, mounting, multiplication, productiveness,
   proliferation, raise, raising, rise, snowballing, splay, spread,
   spreading, surge, swelling, tumescence, up, upping, upsurge,
   upswing, uptrend, upturn, waxing