[syn: addition, add-on, improver]
The Collaborative International Dictionary of English v.0.48:
Improver \Im*prov"er\, n.
   One who, or that which, improves.
   [1913 Webster]
WordNet (r) 3.0 (2006):
improver
    n 1: someone devoted to the promotion of human welfare and to
         social reforms [syn: humanitarian, do-gooder,
         improver]
    2: a component that is added to something to improve it; "the
       addition of a bathroom was a major improvement"; "the
       addition of cinnamon improved the flavor" [syn: addition,
       add-on, improver]