The Collaborative International Dictionary of English v.0.48:

Broaden \Broad"en\, v. t. [imp. & p. p. Broadened; p. pr. &
   vb. n. Broadening.] [From Broad, a.]
   To grow broad; to become broader or wider.
   [1913 Webster]

         The broadening sun appears.              --Wordsworth.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Broaden \Broad"en\, v. t.
   To make broad or broader; to render more broad or
   comprehensive.
   [1913 Webster]

WordNet (r) 3.0 (2006):

broaden
    v 1: make broader; "broaden the road"
    2: 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]
    3: vary in order to spread risk or to expand; "The company
       diversified" [syn: diversify, branch out, broaden]
       [ant: narrow, narrow down, specialise, specialize]
    4: become broader; "The road broadened"

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "broaden":
   accrue, accumulate, add to, advance, aggrandize, amplify,
   appreciate, augment, balloon, bloat, blow up, boom, boost, breed,
   build, build up, bulk, bulk out, crescendo, deal in generalities,
   deepen, develop, dilate, distend, ecumenicize, enlarge, exalt,
   expand, extend, fatten, fill out, gain, gain strength, generalize,
   get ahead, globalize, go up, greaten, grow, hike, hike up, huff,
   increase, inflate, intensify, jack up, jump up, lengthen, magnify,
   make a generalization, maximize, mount, multiply, open, outspread,
   outstretch, parlay, proliferate, puff, puff up, pump, pump up,
   put up, pyramid, raise, rarefy, rise, run up, shoot up, snowball,
   spread, spread out, strengthen, stretch, sufflate, swell, thicken,
   tumefy, universalize, up, wax, widen