Quoth Wikipedia:
A tank destroyer or tank hunter is a type of armoured fighting vehicle, armed with a direct-fire artillery gun or missile launcher, with limited operational capacities and designed specifically to engage enemy tanks.
and
Except for most American designs, tank destroyers were all turretless and had fixed or casemate superstructures.
This makes me wonder: how could a vehicle which mounted its cannon within a turret be differentiated from simply a tank? What other design features qualified the American "tank destroyers" to fall under the definition of "tank destroyer" given in the first quote and excluded them from fulfilling the - rather broad - definition of merely a "tank". Was their classification as tank destroyers purely from the perspective of battlefield role - i.e. these vehicles, even if falling under the technical definition of a tank, were intended to engage enemy armor because their design, though not qualitatively excluding them from the "tank" category, quantitatively made them a more economical choice for engaging enemy armor than other combat vehicles? (Compare how the German Tiger II, a "heavy tank", would mostly be assigned to operate against enemy armor, rather than infantry and lighter vehicles, making its role that of a tank destroyer - but its design was still that of a tank.)
The American tank destroyers were considered to be such because their primary role was to fight tanks. Under their doctrine, tanks were primarily to be used in offensive roles; supporting infantry attacks, and used en masse to exploit the results of such attacks. Meanwhile, tank destroyers were to be used to blunt enemy tank attacks. This doctrine was based on observations of the German blitzkrieg through Europe in 1939 and 1940. To the Americans, this seemed to be based around the use of massed tank formations to rapidly disrupt enemy positions. Infantry units could not effectively combat such units alone - while they could be issued anti-tank weapons, they didn't have the mobility needed to respond to a fast-moving assault. As a result, fast armoured units had to be formed, so they could be rushed into position to block a threatened breakthrough. The Armored Branch, which had control of the US Army's tanks, was not interested in such a role. They were more focused on offensive operations, and had no desire to be trapped in a defensive role. Instead, Tank Destroyer units were formed as part of the Infantry Branch.
American Tank Destroyer units came in two types: towed and mobile. Towed tank destroyers units used towed AT guns, and were to be deployed in front of an armoured attack. Mobile tank destroyers were seen to fill a counterattacking role. They would wait for an armoured attack to develop, and then strike, in battalion or greater strength, at critical points. As such, mobility was emphasised, especially given the need to be moved into positions where an attack was developing. While early TDs, like the M3 GMC, had limited gun traverse, this was quickly found to be unsuitable for the intended tactics. Using a turret let the TDs fight from hull-down positions, and gave more tactical flexibility.
As it happened, the US Army rarely faced the massed tank attacks it expected. As a result, the TD units found themselves redundant for the most part. Instead of being used for their intended role, they were used to support infantry attacks, or deployed in offensives along with the armour.
I have an hour long video on the subject, and the first part of it addresses the question of "just what is a tank destroyer anyway." https://m.youtube.com/watch?v=7ho8TU_JpoI (I have also just sent my first book on TDs to the printer, so I have some knowledge of the subject!)
The thing to note primarily is that a tank destroyer is a piece of equipment defined by the role, not by the design of the equipment itself. A towed 3" AT gun in the US was a tank destroyer, a towed 57mm AT gun was an AT gun. An M10 in the US Army is a Tank Destroyer. An M10 in the British Army is a self-propelled anti-tank gun. It is more than a simple nomenclature change, it is a different mode and expectation of use of that piece of equipment.
For the US Army, the role of the TD required a piece of equipment which at least had the anti-tank capability of a tank, but with far better mobility than a tank. And should be cheaper than a tank. This meant reduced weight, and Tank Destroyer Command was pretty focused on what became M18. Everything else, the M3, the M10, they were expedient vehicles which would have to do until the ideal equipment showed up.
There was some concern about the overlap in vehicle design between TDs and tanks. There was a letter from Tank-Automotive Command about the T49 then under construction, which had a bow machinegun at least, commenting that it didn't matter if the thing was called a Tank, Tank Destroyer or a Tahootie, what mattered was how good it was at doing what it was supposed to do. The T-AC letter, written by the TD Liaison, observed that if it ended up that they were building something better suited to being a tank, then they would happily call it a tank, and continue to fight the war.
There was also a request from the ETO which requested improving the M18 with a bow, coax machine gun, and a few other things. TD Command looked at it, and decided "if we do that, we have just made a tank".
So, the bottom line, a TD in the US Army was a piece of equipment assigned to a unit whose primary role was to react to enemy armored offenses. If it was not assigned to such a unit with that role, it was not a TD.
George Orwell arguing for socialism? I can see I'm going to need to read more, thank you!