Its not just a common word , rather its a name of disease according to mariam Webster dictionary.

Pneumonoultramicroscopicsilicovolcanoconiosis is the word.

pneumonoultramicroscopicsilicovolcanoconiosis refers to a pneumoconiosis caused by inhalation of very fine silicate or quartz dust. It is apprently the longest word in English language.

However, its not used normally in day to day life. A condition meeting the word’s definition is normally called silicosis. This current 45 letter word holds record of being largest word. This word was invented in 1935 by Everett M. Smith, president of the National Puzzlers’ League, at its annual meeting.

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