Here is a cartographic term that has been talked about extensively, but not necessarily defined. Generic. This refers to the descriptor attached to place names. Examples: In Rocky Mountains, the generic is "Mountains". In Rio Grande, it is Rio. "Rio" and "mountain" and "Wadi" and "erg" aren't really cartographic terms in themselves. I believe the US Board on Geographic Names has a list of generics used in the US. I'll see if Icahn track it down.