An eggcorn is a word or phrase that sounds like another word or phrase and is sometimes mistakenly used in place of the latter in a way that seems logical or plausible.
An example might be a writer describing an order to seize and desist rather than the proper term cease and desist, the occasional instance of mixmash instead of mishmash, or cold slaw instead of coleslaw, or someone mishearing the word scapegoat as escape goat.
In these instances the logic comes usually from replacing an unfamiliar word or part with one that might be more familiar. Cease and desist is a legal command that might make one think of having something taken away (seized); mishmash has the notion of a confusing mixture in its definition; and coleslaw invites the interpretation cold slaw since it is served cold.
The term eggcorn—itself an alteration of acorn—is a recent invention, coined by the linguist Geoffrey Pullum to describe the phenomenon.