What is a Paladin?

So, I'm not asking for the obvious, "Oh, they're holy knights, blah blah blah."

No, I mean what is the difference between a Paladin and a Cleric/Fighter? I always felt the Paladin was a redundant class when multiclassing is a core part of the system.

I get in settings/games like Warcraft where by lore they are priests that took up arms and there is no multiclassing but D&D I feel as though Cleric/Fighter fills the trope nicely already.

So, I feel like the reasons for a Paladin's existence must be a lore reason. To be exclusive to humans (except Dorn), this must mean a Paladin is specifically a human type of Cleric/Fighter that receives different training.

I get the Paladin subclasses such as the Inquisitor that is more of a soldier that hunts out heretical mages and such, but the base class and Blackguards feel. . .uneeded?

I love Paladins and Blackguards to death but a Holy Cleric/Fighter and an Unholy Cleric/Fighter seems to fit the trope just the same.

Hell, even normal Clerics are plate wearing, warhammer and shield, face smashers.

So, what is a Paladin and why is it different than a Cleric/Fighter?