"Eggcorn"! An eggcorn is a substitution of a similar-sounding but incorrect word into a phrase that results in a phrase that is still plausible. For example, "ex-patriot" is an eggcorn of "expatriate", because while it is incorrect, it still makes sense. Another is "with baited breath" as a substitution for the correct phrase, "with bated breath". This is different from a malapropism, which is a substitution of a similar-sounding word into a phrase that makes it nonsense.

The term originated in 2003 during an online linguistics discussion, in which one of the participants mentioned a woman who thought the word "acorn" was "eggcorn". One of the other participants suggested using the word to describe the general phenomenon.

I don't know why, but this word and concept is just delightful to me.