In Germany, "tank" is translated to "Panzer", which roughly means armored vehicles. So "Flugabwehrkanonepanzer" (aka: Flakpanzer) is a Panzer.
But a German-flakpanzer is NOT a USA-"tank".
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I think a lot of the confusion is the difference in languages. Panzer =/= tank, its just the closest word between the two languages, so we kinda sorta equate them.
USA's M2 Bradley is "not a tank", but probably would be classified as a German Panzer.
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The "proper name" is "Main Battle Tank", which does refer to a very specific kind of vehicle.
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This is reminding me very much of the "cube rule" for sandwiches. A "hot-dog" isn't a "sandwich", its a "taco" (hot-dogs have the starch covering 3x sides, with the "top" side open).
Other "tacos" include hot-dogs, subs, and slice-of-pies (pies are covered on 3 sides after all)