From 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]
From 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]