If you think you have already heard some long words in English, I can assure you that you haven't heard this one before.
Most long words contain somewhere between 35 and 50 letters. The chemical name of titin, the largest known protein, is made up of 189,819 letters. In fact, it is not just the longest word in English, it takes the record for any language.
If you don't believe me, just listen to all 3 hours, 33 minutes and 23 seconds of this clip of a man reciting the word.
According to Wikipedia, Methionylthreonylthreonylglutaminylarginyl...isoleucine is not technically a word, and its inclusion in a proper dictionary is still being debated. As this protein is a chemical compound, the name given to titin is dubbed a verbal formula.