silencer

Short definition from Wiktionary

Noun

1. 1
   Something that silences or (notionally) nearly silences: An attachment to a gun that reduces the sound it emits.
2. 2
   Something that silences or (notionally) nearly silences: The muffler of an internal combustion engine.
3. 3
   Something that silences or (notionally) nearly silences: Any of various devices to silence the humming noise of telegraph wires.
4. 4
   Something that silences or (notionally) nearly silences: Something that prevents or greatly reduces gene expression.