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pnuemonoultramicroscopicsilicovolcanoconiosis
The longest word may be any of several possible words, ranging fromantidisestablishmentarianism (28 letters)to the coined wordpneumonoultramicroscopic-silicovolcanoconiosis (45 letters, no hyphen)to thecomplete long chemical name for the protein titin (189,819 letters).(see the related question)