Here is the legal definition
Gun Control Act Definitions
Silencer
18 U.S.C., § 921(A)(24)
The term “Firearm Silencer” or “Firearm Muffler” means any device for silencing, muffling, or diminishing the report of a portable firearm, including any combination of parts, designed or redesigned, and intended for the use in assembling or fabricating a firearm silencer or firearm muffler, any part intended only for use in such assembly or fabrication.