The Collaborative International Dictionary of English v.0.48:

Sight \Sight\ (s[imac]t), n. [OE. sight, si[thorn]t, siht, AS.
   siht, gesiht, gesih[eth], gesieh[eth], gesyh[eth]; akin to D.
   gezicht, G. sicht, gesicht, Dan. sigte, Sw. sigt, from the
   root of E. see. See See, v. t.]
   1. The act of seeing; perception of objects by the eye; view;
      as, to gain sight of land.
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            A cloud received him out of their sight. --Acts. i.
                                                  9.
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   2. The power of seeing; the faculty of vision, or of
      perceiving objects by the instrumentality of the eyes.
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            Thy sight is young,
            And thou shalt read when mine begin to dazzle.
                                                  --Shak.
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            O loss of sight, of thee I most complain! --Milton.
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   3. The state of admitting unobstructed vision; visibility;
      open view; region which the eye at one time surveys; space
      through which the power of vision extends; as, an object
      within sight.
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   4. A spectacle; a view; a show; something worth seeing.
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            Moses said, I will now turn aside and see this great
            sight, why the bush is not burnt.     --Ex. iii. 3.
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            They never saw a sight so fair.       --Spenser.
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   5. The instrument of seeing; the eye.
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            Why cloud they not their sights?      --Shak.
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   6. Inspection; examination; as, a letter intended for the
      sight of only one person.
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   7. Mental view; opinion; judgment; as, in their sight it was
      harmless. --Wake.
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            That which is highly esteemed among men is
            abomination in the sight of God.      --Luke xvi.
                                                  15.
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   8. A small aperture or optical device through which objects
      are to be seen, and by which their direction is settled or
      ascertained; -- used on surveying instruments; as, the
      sight of a quadrant.
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            Thier eyes of fire sparking through sights of steel.
                                                  --Shak.
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   9. An optical device or small piece of metal, fixed or
      movable, on the breech, muzzle, center, or trunnion of a
      gun, or on the breech and the muzzle of a rifle, pistol,
      etc., by means of which the eye is guided in aiming. A
      telescope mounted on a weapon, such as a rifle, and used
      for accurate aiming at distant targets is called a
      telescopic sight. --Farrow.
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   10. In a drawing, picture, etc., that part of the surface, as
       of paper or canvas, which is within the frame or the
       border or margin. In a frame or the like, the open space,
       the opening.
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   11. A great number, quantity, or sum; as, a sight of money.
       [Now colloquial]
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   Note: Sight in this last sense was formerly employed in the
         best usage. "A sight of lawyers." --Latimer.
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               A wonder sight of flowers.         --Gower.
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   At sight, as soon as seen, or presented to sight; as, a
      draft payable at sight: to read Greek at sight; to shoot a
      person at sight.

   Front sight (Firearms), the sight nearest the muzzle.

   Open sight. (Firearms)
       (a) A front sight through which the objects aimed at may
           be seen, in distinction from one that hides the
           object.
       (b) A rear sight having an open notch instead of an
           aperture.

   Peep sight, Rear sight. See under Peep, and Rear.

   Sight draft, an order, or bill of exchange, directing the
      payment of money at sight.

   To take sight, to take aim; to look for the purpose of
      directing a piece of artillery, or the like.
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   Syn: Vision; view; show; spectacle; representation;
        exhibition.
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The Collaborative International Dictionary of English v.0.48:

Peep \Peep\ (p[=e]p), v. i. [imp. & p. p. Peeped (p[=e]pt); p.
   pr. & vb. n. Peeping.] [Of imitative origin; cf. OE. pipen,
   F. piper, p['e]pier, L. pipire, pipare, pipiare, D. & G.
   piepen. Senses 2 and 3 perhaps come from a transfer of sense
   from the sound which chickens make upon the first breaking of
   the shell to the act accompanying it; or perhaps from the
   influence of peek, or peak. Cf. Pipe.]
   1. To cry, as a chicken hatching or newly hatched; to chirp;
      to cheep.
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            There was none that moved the wing, or opened the
            mouth, or peeped.                     --Is. x. 14.
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   2. To begin to appear; to look forth from concealment; to
      make the first appearance; as, the sun peeped over the
      eastern hills.
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            When flowers first peeped, and trees did blossoms
            bear.                                 --Dryden.
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   3. To look cautiously or slyly; to peer, as through a
      crevice; to pry.
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            Peep through the blanket of the dark. --Shak.
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            From her cabined loophole peep.       --Milton.
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   Peep sight, an adjustable piece, pierced with a small hole
      to peep through in aiming, attached to a rifle or other
      firearm near the breech.
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The Collaborative International Dictionary of English v.0.48:

Peep sight \Peep sight\
   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.
   [Webster 1913 Suppl.] peepul

WordNet (r) 3.0 (2006):

peep sight
    n 1: rear gunsight having an adjustable eyepiece with a small
         aperture through which the front sight and the target are
         aligned