The Collaborative International Dictionary of English v.0.48:

Meliorate \Mel"io*rate\ (m[=e]l"y[-o]*r[=a]t), v. t. [imp. & p.
   p. Meliorated (m[=e]l"y[-o]*r[=a]`t[e^]d); p. pr. & vb. n.
   Meliorating.] [L. melioratus, p. p. of meliorare to
   meliorate, fr. melior better; akin to Gr. ma^llon rather,
   ma`la very. Cf. Ameliorate.]
   To make better; to improve; to ameliorate; to soften; to make
   more tolerable.
   [1913 Webster]

         Nature by art we nobly meliorate.        --Denham.
   [1913 Webster]

         The pure and benign light of revelation has had a
         meliorating influence on mankind.        --Washington.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Meliorate \Mel"io*rate\, v. i.
   To grow better.
   [1913 Webster]

WordNet (r) 3.0 (2006):

meliorate
    v 1: to make better; "The editor improved the manuscript with
         his changes" [syn: better, improve, amend,
         ameliorate, meliorate] [ant: aggravate, exacerbate,
         exasperate, worsen]
    2: get better; "The weather improved toward evening" [syn:
       better, improve, ameliorate, meliorate] [ant:
       decline, worsen]

Moby Thesaurus II by Grady Ward, 1.0:

137 Moby Thesaurus words for "meliorate":
   accommodate, acculturate, adapt, adjust, advance, air, alter,
   ameliorate, amend, be changed, be converted into, be renewed,
   better, boost, bottom out, break, break up, bring forward, change,
   checker, chop, chop and change, civilize, come about, come along,
   come around, come on, come round, convert, deform, degenerate,
   denature, descant, deteriorate, develop, deviate, diapason,
   diverge, diversify, edify, educate, elevate, emend, enhance,
   enlighten, enrich, fatten, favor, fit, flop, forward, foster, gain,
   gain ground, get ahead, get along, go ahead, go forward,
   go straight, graduate, grow better, haul around, improve,
   improve upon, jibe, lard, lay, lift, look up, make an improvement,
   make headway, make progress, make strides, measure, melodia, mend,
   mitigate, modify, modulate, mutate, nurture, overthrow, perk up,
   pick up, progress, promote, qualify, raise, re-create, realign,
   rebuild, reconstruct, redesign, refine upon, refit, reform, remake,
   renew, reshape, restructure, revamp, revive, ring the changes,
   shape up, shift, shift the scene, show improvement,
   shuffle the cards, skyrocket, socialize, straighten out, strain,
   subvert, swerve, tack, take a turn, take off, transfigure,
   transform, tune, turn, turn aside, turn into, turn the corner,
   turn the scale, turn the tables, turn the tide, turn upside down,
   undergo a change, upgrade, uplift, vary, veer, warble, warp,
   work a change, worsen