What is the longest word in English?

The answer splits by what you accept as a word. The longest entry in major dictionaries is pneumonoultramicroscopicsilicovolcanoconiosis, 45 letters, a lung disease caused by inhaling fine silica dust, and it is somewhat notorious because it was coined in the 1930s largely for the purpose of being the longest word, then picked up by dictionaries after it circulated. Medical writers generally use silicosis. Antidisestablishmentarianism, 28 letters, is usually offered as the longest word with a genuine ordinary history, referring to nineteenth-century opposition to removing the Church of England's official status. Floccinaucinihilipilification, 29 letters, meaning the act of estimating something as worthless, appears in the Oxford English Dictionary and is used almost exclusively in discussions of long words.

Technical vocabulary breaks the contest entirely. Chemical names can run to tens of thousands of letters, since the full systematic name of a large protein simply describes every component in sequence, and the full name of the protein titin runs to roughly 189,000 letters, which most authorities decline to count as a word at all. German-style compounding is not available in English the same way, though English does form long compounds. If you want a defensible single answer for trivia purposes, use pneumonoultramicroscopicsilicovolcanoconiosis for dictionary-longest and antidisestablishmentarianism for longest in real use, and be ready for someone to raise the titin objection.