The longest word in popular usage is ANTIDISESTABLISHMENTARIANISM (a movement opposed to the separation of church and state) at 28 letters.
The 45 letter word:
NEUMONOULTRAMICROSCOPICSILICOVOLCANOCONIOSIS
(a lung disease caused by breathing in certain particles) also appears in many English dictionaries. However, as it's use is strictly medical or scientific, some may not consider this truely part of the English language.
