ˈ ̷ ̷ˌ ̷ ̷ noun
1. : the lens or combination of lenses at the eye end of an optical instrument (as a telescope) through which the image is viewed — see prism binocular illustration
2.
a. : a piece of transparent material (as mica) that resists intense heat and that is mounted and fitted in the side of a furnace to permit a view of the interior
b. : a piece of transparent material (as glass) that is mounted and fitted in the facepiece of a gas mask or other respirator to permit vision