Sounds like a case of two nations (or now I'm posting, three nations, divided by a common language)

Where I come from it's a floor plate magazine when the bottom of the magazine is flush with the bottom of the rifle and is hinged to allow bullet removal.

OR a fixed plate magazine when the bottom of the mag is flush with the bottom of the rifle and is fixed and rounds have to be cycled through the bolt for removal.

OR an extended magazine where the mag extends below the line of the rifle and may or may not be hinged for bullet removal.

OR a removeable magazine where the entire mag bay be removed with it's contents intact.

To illustrate my definition, here's a picture of my non mauser (sorry guys!) with an extended magazine which incidentally does have a hinged floorplate. To compensate for my sin of posting a non mauser rifle here, I'll admit to just having bought a new custom built .404 Jeffery by Sabi Rifles (WITH A MAUSER ACTION!), mercury recoil arrestor and floor plate magazine to use as a DG loaner to clients! - Just have to do the paperwork now!...... some might consider that a little over the top for a loaner, but I figure a client pays a shed load of money for a safari and if he needs a loaner, he should have a good 'un. It's also topped off with QD mounts and a Swarovski 1.5 - 6 x something or other.

If it's of any interest, the extended mag was made for me by a good friend and hunting buddy and it's made the world of difference to the rifle...... it even carries better now than it did with the standard floor plate magazine......