According to Wikipedia, the longest word in any of the major English language dictionaries ispneumonoultramicroscopicsilicovolcanoconiosis, a word which refers to a lung disease contracted from the inhalation of very fine silica particles, specifically from a volcano.
The Oxford English Dictionary containspseudopseudohypoparathyroidism (30 letters), which is an inherited disorder, named for its similarity to pseudohypoparathyroidism in presentation. The term pseudopseudohypoparathyroidism is used to describe a condition where the individual has the phenotypic appearance of pseudohypoparathyroidism type 1a, but is biochemically normal.
The longest non-technical word in major dictionaries isfloccinaucinihilipilification at 29 letters. Consisting of a series of Latin words meaning "nothing" and defined as "the act of estimating something as worthless"...
A real tongue-twister, huh? (^_^) hehe