From The Bang Switch:

Here’s a not so well kept secret, there’s no such thing as an 80% lower in the eyes of the BATF — it’s either a gun or it isn’t. The phrase “80% lower” was coined by a cleaver marketer of incomplete AR lowers years ago and it stuck.

The actual regulation in the Gun Control Act that all this hubbub is based on states, “(A) any weapon (including a starter gun) which will or is designed to or may readily be converted to expel a projectile by the action of an explosive; (B) the frame or receiver of any such weapon.”