A ghost gun is a term for a homemade or improvised firearm that lacks commercial serial numbers or a commercial firearm that has had its serial number removed.
Such weapons are assembled by individuals from parts or kits or that include one unfinished piece — typically the frame or receiver — that requires the purchaser to do some drilling to make the gun fully functional. Because of a loophole in federal gun regulations, these DIY weapons don’t need to have serial numbers, and the kit or individual pieces can be sold without the background check that someone who purchased a gun from a federally licensed dealer would have to undergo.
The term is used mostly in the United States by gun control advocates, gun rights advocates, law enforcement, and some in the firearm industry.