eggcorn, n. Mark Liberman of Language Log coined this one in 2003. An eggcorn is an alteration of a word or phrase through mishearing or reinterpretation. Thus acorn becomes eggcorn. An eggcorn differs from a folk etymology in that it is an error of a few people, and has not, yet, become a standard part of the lexicon.
I would delete the “error of a few people” part—some eggcorns are used by lots and lots of people—and just say “in that it has not become a standard part of the lexicon.” That is, after all, what defines it.