1. Longest word by letter count:
* "pneumonoultramicroscopicsilicovolcanoconiosis" (45 letters) is the longest word in the Merriam-Webster dictionary, meaning "a lung disease caused by the inhalation of very fine silica dust."
2. Longest word by definition:
* "Floccinaucinihilipilification" (29 letters) is often considered the longest word in the English language, meaning "the act of estimating something as worthless."
While it's shorter than "pneumonoultramicroscopicsilicovolcanoconiosis," it's considered by some to be the longest because it's a legitimate word with a clear definition, whereas "pneumonoultramicroscopicsilicovolcanoconiosis" is a made-up word used for its sheer length.
There are other contenders for the "longest word" title depending on how you define "word" and "legitimate." For example, some consider chemical names like "titin" (189,819 letters) to be the longest words, though they're often considered more like technical terms than everyday words.
Ultimately, the answer to this question depends on your definition of "longest word."