The part of the frame in the front that starts where the trigger loop ends and goes to the very front. It covers where the spring and guide rod are in the slide. The rail is either mounted or molded to this part.
Actually it depends on the weapon and nomenclature. There is also a dust cover on an M-16A1 and most variants that snaps over the ejection port to keep crud out of the bolt assembly and chamber area.
There are others.
Geoff
Who likes to confuse things when it comes to nomenclature.