Here are a few contenders:
1. Pneumonoultramicroscopicsilicovolcanoconiosis: This 45-letter behemoth refers to a lung disease caused by inhaling very fine silica dust. It's often cited as the longest word in the English language, but it was coined in 1935 by American physician and word enthusiast, E. W. Pringle.
2. Floccinaucinihilipilification: This 29-letter word means "the act of estimating something as worthless". While less common, it's a legitimate word found in dictionaries.
3. Antidisestablishmentarianism: This 28-letter word refers to opposition to the separation of church and state.
4. Longest words in other languages: Languages other than English boast even longer words. For example, in German, "Rindfleischetikettierungsüberwachungsaufgabenübertragungsgesetz" (63 letters) is a law about the labeling of beef.
In Conclusion:
While Pneumonoultramicroscopicsilicovolcanoconiosis is the most widely known "long word," it's a fabricated term. "Floccinaucinihilipilification" is generally considered the longest legitimate word in the English language. However, you'll find even longer words in other languages.