An Etymological Dictionary of Astronomy and Astrophysics

Minute images or replicas of objects supposed by ancient atomist philosophers to be shed from any object and used in the explanation of vision. According to Democritus (c. 460-c. 370 BC) and Epicurus (341-270 BC), these replicas or effigies, called eidola, were perpetually peeled off
the surfaces of things and caused vision by entering in the eye.

See also: L. translation of eidola by Lucretius (1st Century BC), from L. simulacrum “likeness, image,” from simulare “to → simulate.”