The absolute longest word in the world is the chemical name for the human protein titin. It contains 189,819 letters and takes over three hours to pronounce. [1, 2, 3]

Because this chemical name is practically unreadable, the longest words are usually categorized by what you consider an “official” word:

• In English Dictionaries: The longest word found in major dictionaries like Merriam-Webster is Pneumonoultramicroscopicsilicovolcanoconiosis (45 letters). It refers to a lung disease caused by inhaling very fine silicate or quartz dust. [1, 2]
• Longest Non-Technical Word: Floccinaucinihilipilification (29 letters), which means the act or habit of estimating something as worthless. [1, 2]
• Longest Word in Literature: The ancient Greek comedic playwright Aristophanes created the word Lopadotemachoselachogaleokranioleipsanodrimhypotrimmatosilphiokarabomelitokatakechymenokichlepikossyphophattoperisteralektryonoptekephalliokigklopeleiolagoiosiraiobaphetraganopterygon. It has 171 letters (or 182, depending on the transliteration) and represents a fictional, extravagant stew.[1, 2]

For a closer look at the technical term for titin and other massive linguistic anomalies, you can explore the Merriam-Webster Long Words List. [1, 2]