The Collaborative International Dictionary of English v.0.48:

Widen \Wid"en\, v. i.
   To grow wide or wider; to enlarge; to spread; to extend.
   [1913 Webster]

         Arches widen, and long aisles extend.    --Pope.
   [1913 Webster]

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):

widen
    v 1: become broader or wider or more extensive; "The road
         widened" [ant: contract, narrow]
    2: make (clothes) larger; "Let out that dress--I gained a lot of
       weight" [syn: let out, widen] [ant: take in]
    3: make wider; "widen the road"
    4: extend in scope or range or area; "The law was extended to
       all citizens"; "widen the range of applications"; "broaden
       your horizon"; "Extend your backyard" [syn: widen,
       broaden, extend]

Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "widen":
   accrue, accumulate, add to, advance, aggrandize, amplify,
   appreciate, augment, balloon, bloat, blow up, boom, branch,
   branch out, breed, broaden, build, build up, bulk, bulk out,
   crescendo, deal in generalities, deepen, deploy, develop, dilate,
   disperse, distend, ecumenicize, enlarge, expand, extend, fan,
   fan out, fill out, flare, gain, gain strength, generalize,
   get ahead, globalize, go up, greaten, grow, hike, hike up, huff,
   increase, inflate, intensify, magnify, make a generalization,
   mount, multiply, open, open up, outspread, outstretch, overgrow,
   overrun, proliferate, puff, puff up, pump, pump up, raise, ramify,
   rarefy, rise, run up, shoot up, snowball, splay, spraddle,
   sprangle, sprawl, spread, spread like wildfire, spread out,
   strengthen, stretch, sufflate, swell, tumefy, unfold, universalize,
   up, wax