The noun eggcorn denotes an alteration of a word or phrase through the mishearing or reinterpretation of one or more of its elements as a similar-sounding word.

It was coined by Geoffrey Keith Pullum (born 1945), Professor of General Linguistics, with reference to a misinterpretation of acorn as egg corn; it first appeared on the website Language Log in 2003:

Egg corns: folk etymology, malapropism¹, mondegreen², ???
Chris Potts has told me about a case in which a woman wrote “egg corns” for “acorns”. This might be taken to be a folk etymology, like “Jerusalem” for “girasole” in “Jerusalem artichoke” (a kind of sunflower). But it might also be treated as something like a mondegreen […], the kind of “slip of the ear” that is especially common in learning songs and poems. Finally, it’s also something like a malapropism, where a word is mistakenly substituted for one of similar sound shape.
Although the example is somewhat like each of these three named categories of errors, it’s not exactly any of them. Can anyone suggest a better term?
At greater length:
It’s not a folk etymology, because this is the usage of one person rather than an entire speech community.
It’s not a malapropism¹, because “egg corn” and “acorn” are really homonyms (at least in casual pronunciation), while pairs like “allegory” for “alligator”, “oracular” for “vernacular” and “fortuitous” for “fortunate” are merely similar in sound (and may also share some aspects of spelling and morphemic content).
It’s not a mondegreen² because the mis-construal is not part of a song or poem or similar performance.
Note, by the way, that the author of this mis-hearing may be a speaker of the dialect in which “beg” has the same vowel as the first syllable of “bagel”. For these folks, “egg corn” and “acorn” are really homonyms, if the first is not spoken so as to artificially separate the words.
Posted by Mark Liberman at September 23, 2003 12:33 PM
[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 “spoonerism³” is not appropriate.]

¹ A malapropism is the mistaken use of a word in place of a similar-sounding one, as in to dance a flamingo instead of flamenco.
² A mondegreen is a misunderstood or misinterpreted word or phrase resulting from a mishearing of the lyrics of a song.
³ A spoonerism is a verbal error in which a speaker accidentally transposes the initial sounds or letters of two or more words—cf. also marrowsky.

This folk-etymological alteration of acorn is first recorded in a letter dated 16^th June 1844 that Samuel G. McMahan, an American frontiersman, wrote from Oregon to John Marsh, a California pioneer:

I am bungler at [= at writing], but what you cant read mayby you caan gess at it. I hope you are as harty as you ust to be and that you have plenty of egg corn bread which I cann not get her [= here] and I hope to help you eat some of it soon.

Jeremy Butterfield, editor of the 2015 edition of Fowler’s Dictionary of Modern English Usage, explains:

Try saying ‘acorn’ in a slow, southern-US-states drawl, pronouncing the c like a g, and what you come out with will sound like ‘egg-corn’.

I have found an interesting example in the Argus-Leader (Sioux Falls, South Dakota) of Sunday 5^th November 1967; a portrait of the American actress Barbara Anderson (born 1945), who played policewoman Eve Whitfield in the television series Ironside, contains:

“I couldn’t say acorn in the ‘Ironside’ pilot,” Barbara noted. “It kept coming out ‘eggcorn.’ Finally, they just left it in.”

When eggcorn was entered into the Oxford English Dictionary (OED) in September 2010, the quarterly update for the online revision of this dictionary included the following note:

As early as 1844, people were reinterpreting the word “acorn” as “eggcorn”, either deliberately, for humorous purposes, or in all innocence, in a struggle to analyse, in a way that made sense to them, what the word’s spelling must be: acorns are, after all, seeds which are somewhat egg-shaped, and in many dialects the formations acorn and eggcorn sound very similar. Since 2003, it has become a widely accepted term for this category of words as a whole, appearing in books and journals, and on the internet, often alongside its musical sibling, the mondegreen or misheard lyric (which first appeared in the OED in 2002). As such, it has now become an autological word: one which belongs to the category it describes.

In the above-mentioned dictionary, Jeremy Butterfield explains that what distinguishes eggcorns from folk etymologies is that the latter become a norm and are collective, whereas the former are individual—which does not prevent them from recurring individually time and time again. However, eggcorns can develop their own folk etymologies; the British writer Jeanette Winterson (born 1959) gave an example of this in 2006 (in the phrase to give up the ghost, meaning to expire, ghost denotes the soul as the principle of life):

The other day my elderly country neighbour asked for a bit of help to get his new washing machine into the kitchen. That generation never use ‘it’, always, ‘he’ or ‘she’, so I wasn’t surprised to hear the washing machine called ‘he’, but I was surprised by what followed: “My old washing machine, he’s given up the goat,” he said, in a broad Gloucestershire accent.
“The goat?” I replied, “are you sure?”
“Oh yes,” said my neighbour, “aint you never heard that expression before, given up the goat?”
“Well, not exactly… where does it come from?”
“Ah well,” said my neighbour, “in the old days, when folks didn’t have much, and mainly worked the land, a man would set store by his animals, especially his goat, and when he come to die, he would bequeath that goat to his heirs, and that is why we say, ‘he’s given up the goat’.”

As a conclusion, here is another passage from the above-mentioned dictionary edited by Jeremy Butterfield:

Eggcorns explain phrases that can, at first sight, look like bizarre mistakes, such as ‘to all intensive purposes’, ‘the Delhi lama’, ‘the Dahlia Lama’, (Dalai Lama), ‘Asparagus syndrome’ (Asperger’s syndrome), ‘above/beyond approach’ (above /beyond reproach), and countless others. Once one knows about eggcorns, it can be entertaining to listen and look out for them. To do so is even, perversely, a way of celebrating the playfulness of language. My favourites include ‘to have a poncho’ for something, ‘the Nuke of the North’ (Nanook of the North), ‘hairy-fairy’ and ‘to go off on a tandem’. While eggcorns do cause mirth, it would be condescending to be superior and view them as signs of poor literacy. Many fit specific phonetic patterns, and illustrate systematic phonetic trends. They also display people’s intelligence and ingenuity in making sense of what they hear, which is, after all, what we do all the time with any speech we hear.