Panzer only = tank if you're a naive English Tommy shouting "oh look mate, it's a panzer!" It is an adjective and can describe motorized infantry divisions, armored (tank) battalions, or even medieval knights wearing chain-maille.
It has literal meaning of "armor" if I remember right. A tank is the ultimate in armored devices, hence why "Panzer" or "armor/armour" makes a particularly apt name for them. Tank only works in English because it was a falsified code name from WWI, otherwise you're gonna get tanks called "barrels" in other languages :p
