noun

• 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.

• 2Entomology
  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.

• 4Zoology
  The (usually transparent) scale that covers the eye in snakes and certain other reptiles.