The longest word to appear in standard English dictionaries is: pneumonoultramicroscopicsilicovolcanoconiosis (45 letters). This is the name of a lung disease suffered by miners. It first appeared in Webster's New International Dictionary and then later in the second edition of the Oxford English Dictionary.

There are quite a few other very long words of a technical or scientific nature, but these do not appear in standard dictionaries. The longest of these is the term for the formula C1289H2051N343O375S8, which has 1,913 letters.