The Collaborative International Dictionary of English v.0.48:

eye shadow \eye" shad`ow\, eyeshadow
\eye"shad`ow\([imac]"sh[a^]d`[=o]), n.
   a cosmetic powder or cream of a dark tint, applied on the
   eyelids or just under the eyebrows to accentuate the eyes.
   Compare eyeliner.
   [PJC]

WordNet (r) 3.0 (2006):

eyeshadow
    n 1: makeup consisting of a cosmetic substance used to darken
         the eyes