What is the term or word that describes made up words?

neologism..

A neologism (from Greek νεολογισμός "νέος" [neos] = new; "λόγος" [logos] = word) is a word, term, or phrase which has been recently created ("coined") — often to apply to new concepts, to synthesize pre-existing concepts, or to make older terminology sound more contemporary. Neologisms are especially useful in identifying inventions, new phenomena, or old ideas which have taken on a new cultural context. The term e-mail, as used today, would be an example of a neologism.

The closest match is, 'Undefined'. Well, the problem with that is that any made-up word is no more and no less still, a word and this is because all words were made up by someone at some point (mostly Shakespeare and the Greeks, but many other people too). What I think you are getting at is whether its defined in a dictionary or not. Seeing as you asked the question in English, the English dictionary in particular. This depends on whether the word is accepted by the people that govern the english language at Oxford University. So, if this is what you mean then you could say that you invented a new word, that the word isn't defined in the Dictionary or it might be simpler to describe the word as 'undefined'. (Edit) Someone above suggested 'neologism'. That's the best answer, I reckon.

The Simpsons, an animated television series, has used and coined many neologisms for humorous effect, many of which are only used once. The most famous example is Homer Simpson's signature annoyed grunt, "D'oh!" which has been listed in the Oxford English Dictionary, in addition to smaller references such as OUP's single-volume Oxford Dictionary of English (second edition).

Few of the following would qualify as neologisms from a strict lexicological perspective due to their extremely limited uses outside of the show. For those that have found their way into regular use, the route passes through the considerable fan-base where use of these words carries the prestige of pop-culture literacy among those who catch the references, just as among other cultural groups a clever parallel to a well-known phrase from the literary or rhetorical canon would be acknowledged.

The following is presented, then, as a glossary of words or phrases invented by the show which one or more characters use in regular speech, as though intended as real terms. This does not include names of invented characters, locations, or products.

Made Up Word

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I believe the word you're looking for is "cromulent". It was used to describe the made up word "embiggens"

Bushism - named after george bush using words that dont exist

portmanteau

It takes two existing words and combined them into one

chortle from chuckle and snort, coined by Lewis Carroll

gibberish

balderdash

gobbledeegook