Here are a few possibilities:
* Longest word in the English dictionary: This title goes to pneumonoultramicroscopicsilicovolcanoconiosis, a lung disease caused by inhaling very fine silica dust. It's a whopping 45 letters long!
* Longest word in the English language: Technically, there's no limit to how long a word can be in English, as new words are constantly being created. So, the longest word could be a super-long scientific term, a made-up word, or even a sentence used as a single, very long word!
* Longest word in a single language: This depends on which language you're looking at. For example, the Finnish word "lehtipuuttalankaverkkokauppa" (meaning "leaf tree wire online store") is 29 letters long.
* Longest word without repeating letters: This record belongs to "uncopyrightable" with 15 letters.
So, the answer to your question really depends on how you define "longest word"!