The Collaborative International Dictionary of English v.0.48:

Ocellus \O*cel"lus\, n.; pl. Ocelli. [L., dim. of oculus an
   eye.] (Zool.)
      (a) A little eye; a minute simple eye found in many
          invertebrates.
      (b) An eyelike spot of color, as those on the tail of the
          peacock.
          [1913 Webster]

WordNet (r) 3.0 (2006):

ocellus
    n 1: an eye having a single lens [syn: simple eye, stemma,
         ocellus]
    2: an eyelike marking (as on the wings of some butterflies);
       usually a spot of color inside a ring of another color [syn:
       eyespot, ocellus]