noun. A series of words that result from the misunderstanding of a word or phrase as some other word or phrase having a plausible explanation, as free reign for free rein, or to the manor born for to the manner born (from William Shakespeare's Hamlet).

noun. An idiosyncratic but semantically motivated substitution of a word or phrase for a word or words that sound identical, or nearly so, at least in the dialect the speaker uses.

"(NY Times article link on the Language Log) The key to an eggcorn is that it is a mis-hearing by a native speaker that has its own internal logic."

folk etymology  malapropism  mondegreen