What does "silencer" mean?

Silencer: (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.