In firearms terminology, the receiver is the part of a firearm that houses the operating parts. It is sometimes called the body of the firearm, and especially in the context of handguns (revolvers and pistols) it is often called the frame. It is often made of forged or stamped steel or aluminum; in addition to these traditional materials, modern materials science and materials engineering have introduced polymers and sintered metal powders to receiver construction.
In strictly legal terms, in the United States the receiver is the actual firearm itself, and as such it is the controlled part (without which operating is impossible). Generally, the law views the receiver as that part of a firearm housing that has the serial number upon it. Thus, in the case that a firearm has multiple receivers (such as the AR-15, which has an upper and lower receiver) the legally controlled one is the one that is serialized (the lower, in the AR-15's case). http://lux-essay.com/