Supercalifragilisticexpialidocious

The longest word in the world is the English term: "methionylthreonylthreonyl(...)isoleucine".

This term, which has no less than 189, 819 letters, is the chemical name of the largest protein: titin. It would take three hours to recite it from beginning to end!

I didn't know that... but now I do. Thank you!

or pneumonoultramicroscopicsilicovolcanoconiosis (yes, that is a real word and I definitely didn't look it up how to spell it)