You and the guy above have the same avatar so I was super confused for a second hah.

Frame is the firearm, or at least whatever holds the trigger, I am sure the atf has some legal nonsense that specifically defines this, but that is mostly it. You can buy after market slides all day long.

Though, some pistols, like one of the sigs. Actually use a "chassis" as the legal identifiable part. It's not the "grip" portion of the pistol per se, but instead they have a separate mechanical assembly that holds the trigger and slide rails, independent of the grip. this fits into the polymer grip of the sig. That's about the only pistol that I know is a bit different