The Longest Word

The longest word in any of the major English language dictionaries is pneumonoultramicroscopicsilicovolcanoconiosis, a 45-letter word which refers to a lung disease contracted from the inhalation of very fine silicious particles. Research has discovered that this word was originally intended as a hoax. It has since been used in a close approximation of its originally intended meaning, lending at least some degree of validity to its claim.

The Oxford English Dictionary contains pseudopseudohypoparathyroidism (30 letters).

The longest non-technical word in major dictionaries is floccinaucinihilipilification at 29 letters. Consisting of a series of Latin words meaning "nothing" and defined as "the act of estimating something as worthless", its usage has been recorded as far back as 1741.