It all depends on what pistol you are talking about for example on the Revolver it is a grip panel where you hold the revolver, if you are talking about a Semi-Automatic Pistol, there are two things, the grip panel, and the ammo magizine.
The bottom of the grip of a handgun.
The bottom of the grip of a handgun.
The bottom of the pistol grip.
It is called a pistol
Pistol aficionados refer to it as recoil.
It is called a grip.
The link at the bottom of this page will take you to the Ruger website for your pistol. You can download the owner's manual for your pistol, which includes instructions on assembly and maintenance.
It is called the Mossberg 45 Pistol.
Well, there were swords with built in pistols called a Cutlass pistol, there are pocket knives with a single shot pistol, called a knife pistol, and military rifles with a bayonet on them (no special name).
You will need a professional appraisal
A gunsmith
Holster