End milling, flat milling, bottom cleaning == all the same, in my experience. That is, there are probably supposed to be subtle differences, but the manufacturers (and more often, sellers) don't seem to make any distinction, and they use the terms interchangeably.
Note -- a Forstner bit is NOT the same thing. There is a pointy centering tip to a Forstner that is not at all desirable when milling chips. (To my ears "flat-bottom Forstner bit" makes no sense. Is it flat-bottomed, or is it a Forstner bit? Can't be both.)