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Pneumonoultramicroscopicsilicovolcanoconiosis is the longest word entered in the most trusted English dictionaries.
The definition is "a lung disease caused by inhalation of very fine silicate or quartz dust." (Note that it is not entered in the Learner's Dictionary--it is considered too specialized of a term to qualify for entry in this type of dictionary.)
There are other long words that exist, but they are rarely or never used in a sentence. Two examples of words like this are "antidisestablishmentarianism" and "supercalifragilisticexpialidocious."
There is a lake in Massachusetts called Lake Chargoggagoggmanchauggauggagoggchaubunagungamaugg. (It is also sometimes called Webster Lake.)
But the longest string of letters used to describe something is actually the chemical name for a protein, and it has 189,819 letters and takes more than 3 hours to say.