eggcorn
A word or phrase that sounds like and is mistakenly used in a seemingly logical or plausible way for another word or phrase either on its own or as part of a set expression
update (9/30/2003): Geoff Pullum suggests that if no suitable term already exists for cases like this, we should call them "egg corns", in the metonymic tradition of "mondegreen", since the eponymous solution of "malapropism" and "spoonerism" is not appropriate.
Where it comes from
Suggested by British-American linguist Geoffrey K. Pullum following a discussion on the Language Log website on September 23, 2003, by American linguist Mark Liberman, about a woman who had long believed the word acorn to be egg corn.
Adapted from the Wiktionary entry eggcorn (CC BY-SA 4.0).