The Collaborative International Dictionary of English v.0.48:

Silencer \Si"lenc*er\, n.
   One that silences; specif.:
   (a) The muffler of an internal-combustion engine.
   (b) Any of various devices to silence the humming noise of
       telegraph wires.
   (c) A device for silencing the report of a firearm shooting
       its projectiles singly, as a tubular attachment for the
       muzzle having circular plates that permit the passage of
       the projectile but impart a rotary motion to, and thus
       retard, the exploding gases.
       [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Muffler \Muf"fler\, n.
   1. Anything used in muffling; esp., a scarf for protecting
      the head and neck in cold weather; a tippet.
      [1913 Webster]

            Fortune is painted blind, with a muffler above her
            eyes.                                 --Shak.
      [1913 Webster]

   2. (Mus.) A cushion for terminating or softening a note made
      by a stringed instrument with a keyboard.
      [1913 Webster]

   3. A kind of mitten or boxing glove, esp. when stuffed.
      [1913 Webster]

   4. One who muffles.
      [1913 Webster]

   5. (Mach.) Any of various devices to deaden the noise of
      escaping gases or vapors, as a tube filled with
      obstructions, through which the exhaust gases of an
      internal-combustion engine, as on an automobile, are
      passed (called also silencer).
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

silencer
    n 1: short tube attached to the muzzle of a gun that deadens the
         sound of firing
    2: a tubular acoustic device inserted in the exhaust system that
       is designed to reduce noise [syn: silencer, muffler]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "silencer":
   acoustic tile, antiknock, baffler, cushion, damp, dampener, damper,
   gag, hushcloth, muffle, muffler, mute, muzzle, quietener,
   silence cloth, soft pedal, sordine, sordino,
   sound-absorbing material, soundproofing, soundproofing insulation,
   sourdine