From The Collaborative International Dictionary of English v.0.48 :
eyeliner \eye"lin`er\ ([imac]"l[imac]`n[~e]r), n.
a cosmetic powder of a dark tint, applied to the eyelid close
to the base of the eyelashes to accentuate the eyes. Compare
eye shadow.
[PJC]
From WordNet (r) 3.0 (2006) :
eyeliner
n 1: makeup applied to emphasize the shape of the eyes