What is the longest word in major English dictionaries?

The longest word in major English dictionaries is pneumonoultramicroscopicsilicovolcanoconiosis. It has 45 letters and refers to a supposed lung disease associated with inhaling extremely fine silica or volcanic dust.

The word was coined in 1935 by Everett M. Smith, then president of the National Puzzlers’ League. It was deliberately constructed as a record-setting long word, rather than arising naturally through centuries of ordinary use. It later appeared in the 1939 supplement to Merriam-Webster’s New International Dictionary.

Its parts draw from Greek and Latin roots: pneumono- relates to the lungs, ultra- means beyond, micro- and -scopic concern smallness and seeing, silico- refers to silica, volcano- to volcanic material, coni- to dust, and -osis to a medical condition. The word is often treated as a synonym for silicosis, though the specific volcanic-dust disease it names is not a standard established diagnosis.

Long chemical names, including the systematic name associated with titin, can be vastly longer, but they are generally not counted as ordinary dictionary words. That distinction is why this 45-letter coinage remains the conventional quiz answer.