Pure terminology question. Please don’t turn it into a gun control thread.
What is a “receiver”? I’ve seen the term used but never defined. For instance in one thread about dismantling a gun, someone said you can take parts out, but the “receiver” is the firearm for legal purposes.
What does that mean, please?
The “receiver” is the part of the gun that receives the magazine.
What does that mean, please?
I rifle consists of a number of parts:
In most rifles, the receiver connects to the barrel and the trigger assembly and receives the magazine full of ammunition. The loading/ejecting/firing mechanism is either integrated into the receiver or attaches to it.
The wiki for the term is actually pretty good, Piper: Receiver (firearms) - Wikipedia
Basically, it’s the part of the firearm that everything else is connected to. By everything else, I mean things like the barrel, the magazine, grips, stock, sights, bolt. It doesn’t have to be where the chamber is. Sig’s P320 pistol, the receiver is a “fire control group”, that you can change the grips, caliber, magazine, pretty much everything about the pistol, and still have only one ‘gun.’
In an AR-15, the lower assembly, like here: File:Mil-Spec AR-15 lower receiver right side view.jpg - Wikipedia is the receiver. Note the lack of chamber, barrel, magazine, trigger, etc…
TITLE 27 CFR CHAPTER II PART 478 says in part;
In other words the receiver is the part that most of the other major components attach to.
ETA; What Gray Ghost said. 
As for the bit of your question about “for legal purposes”, the “receiver” is the part that you must pass a background check to purchase from an FFL. You can buy a barrel, a pistol grip, a bolt carrier, etc (basically, all the other parts of a firearm) online and have them shipped to your house, but the “receiver” is the part that legally constitutes a firearm, and in order to purchase one, you must abide by the rules of purchasing a firearm in your location.
The receiver is to a firearm what the chassis is to a car.
So what would happen if someone made two separate pieces that could attach together to make a receiver? Like, maybe slice a standard receiver down the centerline, and put in some holes to bolt the two halves together. Is the law defining “receiver” detailed enough that one of those two pieces could be designated “the receiver”, or would both require a license, or neither?
So, ok, there’s the concept of an “80% lower”. This is a machined piece of metal where 80% of the work in turning it into a lower receiver has been completed. It can be legally sold to anyone without a background check or record-keeping.
Then, it seems that in most states it is perfectly legal to manufacture your own firearm, without informing or getting permission from any government agencies. So you can mill out the rest of the lower and you now have a firearm.
Anyways, notably, let’s say you have everything done but installing the trigger springs. This is now a “99%” receiver and is considered a firearm even though it isn’t yet functional.
Anyways, in your example, the two pieces on a table that are ready to snap together are basically a “99% receiver”. You need to do less than 1% of the work to make a receiver from scratch in order to turn it into a receiver. So it’s a firearm.
Also, for better or for worse, this detailed set of guidelines was generated by the federal agency ATF, and is essentially their interpretation of the underlying laws. De facto, though, if you go against their interpretation you are personally in jeopardy of going to prison.
Pretty good explanations so far.
In most cases, at least for pistols, the part that you hold is the “receiver”. The special case of the Sig P320 Fire Control Unit has been addressed.
Another special case is the Ruger Mark series of 22 pistols. The grip/trigger assembly that is the serialized part on most pistols is not on these units. The barrel and breach assembly is instead. So while you can buy Glock slides and barrels all day long with no issues, the similar parts on a Ruger 22 have to go through an FFL.
The two pieces together might well be a receiver (or a 99% receiver). And if I order them both from the same supplier at the same time, it’s clear that that supplier is selling me a receiver (and hence a firearm). But what is each one separately? Presumably, then, each piece on its own would be a 49.5% receiver? If I order a 49.5% left receiver from one supplier, and a 49.5% right receiver from another supplier, which one is “a firearm”? Both, neither?
The short version is that the ATF decides. For example, in the case of the Browning M1919 it’s the left side plate that’s considered the “receiver” for legal purposes. Why not the right side plate? No really great reason, AFAICT. They just needed to pick one. They’re both integral to the proper function of the firearm. Another example is the AR-15. A lower receiver, the legal “receiver”, isn’t much use without an “upper” receiver. They’re both the proper function of the firearm, but because the ATF decided the lower receiver should be the serial-numbered part, you need to meet various legal requirements to acquire one.
