Pneumonoultramicroscopicsilicovolcanoconiosis (45 letters): A lung disease caused by inhaling very fine silica dust, especially from volcanoes.
Pseudopseudohypoparathyroidism (30 letters): A rare genetic disorder affecting the parathyroid glands, resulting in abnormal calcium and phosphate levels in the blood.
Floccinaucinihilipilification (29 letters): The act of assigning very little importance to something.
Antidisestablishmentarianism (28 letters): Opposition to the separation of church and state.
Honorificabilitudinitatibus (27 letters): A Shakespearean word meaning "the state of being deserving of honor."
Incomprehensibilities (21 letters): The state of being impossible to understand.
Strengths (10 letters): Interestingly, this seemingly simple word is the longest word in the English language that is composed entirely of only one vowel and one consonant.
Rhythm (7 letters): Another interesting one, this word is the longest in the English language that does not contain any repeated letters.
Uncopyrightable (17 letters): The inability to be protected by copyright.
Dermatoglyphics (15 letters): The study of fingerprints and other ridges on the palms and soles.