peep sight

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A rear sight of a firearm consisting of an adjustable eyepiece with a small opening through which the front sight and the target are aligned.

from The Century Dictionary.

• noun A plate containing a small hole through which the gunner sights, attached to the breech of a cannon or small arm. See cut under gun.

from the GNU version of the Collaborative International Dictionary of English.

• An adjustable piece, pierced with a small hole to peep through in aiming, attached to a rifle or other firearm near the breech; -- distinguished from an open sight.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun On some rifles, a type of sight where the shooter looks through a small aperture on the rear sight.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun rear gunsight having an adjustable eyepiece with a small aperture through which the front sight and the target are aligned