Suppressor
A device that reduces the audible report and muzzle flash of a firearm.
Also: silencer, can, moderator
A suppressor (legally silencer; in shooter slang can) attaches to the muzzle and slows and cools the propellant gases, reducing the audible signature. They never make a firearm silent — gunshots remain hearing-damaging without ear protection — but they reduce decibels by 20–35 dB. NFA-regulated in the US; legal in most states with the tax stamp and approval.