English is a weird language; we take words from other languages and end up using them in ways that were never intended.
"Jeton" is simply a French word which means roughly the same thing we mean by the word "token", and there are related words in other European languages (such as "żeton" and "zseton") that mean roughly the same thing. So the tokens used in casinos in Europe are called "jetons" by the Europeans because that's the word used in Europe for coin-like objects such as tokens. Meanwhile, in the US, we call the tokens used in casinos "poker chips" because of the historical prominence of poker in American gambling, and even before poker was big we called them "checks" because they represented a debt owed by the casino to the player.
Now, as it turns out, again for historical reasons, the tokens used in casinos in Europe are made of acrylic and come in assorted shapes and sizes, whereas the tokens used in casinos in America are made from a clay-like material and are a uniform shape and size (mostly). In other words, the tokens used in Europe are different from the tokens used in America - although they're used in the very same way for the very same purpose. I wrote a little bit about why they evolved differently
here, and the rest of that thread has some good information about jetons as well.
Once upon a time, if you took a European to an American casino and pointed to the piles of chips and asked him "What are those?" he'd answer "Jetons", meaning coin-like tokens representing monetary value used in circumstances such as within a casino. And if you as an American went to a European casino and asked one of the natives "What are those?" while pointing at the piles of acrylic doo-dads, he'd likewise answer "Jetons". And then if you brought some back and showed them to your friends, and they asked "What are those?" then you'd say "Jetons!" because that's what your friends in Monaco said they were.
In other words - originally, "jeton" just meant "casino token", but eventually, to Americans, "jeton" meant "the kind of casino token that they use in Europe" because a) they look different from the casino tokens we use in America and b) we already have a word for the casino tokens we use in America: poker chips!
There's similar examples for other words. A very old example is "beef" meaning the meat from a cow, whereas in French "boeuf" is both the meat and the animal; we borrowed the word but changed the meaning to be more specific. Something more recent is the word "gelato" which in Italian simply means "ice cream" but in English means "the kind of ice cream that they serve in Italy".
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