Well, "mutant" actually means something specific. Like, how... [oh god what does she actually know about bending] ...only Water Tribe people can be waterbenders, only certain people are mutants, and if you're not born that way, you can't become one.

And--not so sure about this one, 'cause it's not my universe--"meta-human" covers everyone with powers, the same way you're all benders, but you break it down farther by what kind of bender you are.

[Questionable Analogy Time with Nita.]