1Any of various types of part attached to the end of the eyepiece of an optical instrument; specifically (a) a tubular attachment which alters the image that the viewer sees, or which makes viewing more comfortable; (b) a protective cap for the eye lens.
In some small moths: an expanded basal segment of the antenna which partly covers the eye.
3In embalming and funeral practice: a device inserted under the eyelids to maintain the shape of the eyeball and keep the eyelids closed.
The (usually transparent) scale that covers the eye in snakes and certain other reptiles.
Early 19th century. From eye + cap.