Silencer is the legal name for it. No they don't silence a firearm as I can tell you from a great deal of experience, but that is the legal term.
26 USC chapter 44, Title I, 921(24)
The terms "firearm silencer" and "firearm muffler" mean any de- vice for silencing, muffling, or diminish- ing the report of a portable firearm, including any combination of parts, de- signed or redesigned, and intended for use in assembling or fabricating a fire- arm silencer or firearm muffler, and any part intended only for use in such as- sembly or fabrication.
Supressor may be the most accurate term, but it's not what .gov calls it.