What is the longest word?

Pneumonoultramicroscopicsilicovolcanoconiosis has 45 letters and is the longest word in a dictionary.
Titin is the largest known protein and it's scientific name has 189,819 letters. it starts with methionyl.... and ends with isoleucine.

Pneumonoultramicroscopicsilicovolcanoconiosis is not truly the longest as it was deliberately coined to be the longest word in English. Other words that are longer than the one below are either coined deliberately to be longer words (Supercalifragilisticexpialidocious for example) or medical/technical jargon and are simply not used.

The longest non-technical word in common usage and 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. It was actually used in a debate in 1953 by Winston Churchill in the Houses of Parliament in London, Senator Jesse Helms, used this word when referring to the Comprehensive Test Ban Treaty, which Helms deemed worthless, the North Carolina senator reportedly said, "I note your distress at my floccinaucinihilipilification of the CTBT."