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]