The longest English word is 189,819 letters long.

While pneumonoultramicroscopicsilicovolcanoconiosis is the longest word in the English dictionary, an even longer word exists outside the dictionary.

The extended term for "titin" has 189,819 letters, but the first 61 letters are methionylthreonylthreonylglutaminylarginyltyrosylglutamylsery.  It takes three or more hours to pronounce the entire word.  Titin is the body's largest known protein that maintains muscular passive elasticity.

The extended word for titin is so long because it's a technical term for the protein's full chemical composition.  It includes all 244 independent protein domains that form titin, resulting in a sequence of 244 chemical compounds.