Just the facts...

The longest word in the English language, according to the Oxford English Dictionary, is  

"pneumonoultramicroscopicsilicovolcanoconiosis"

This word has 45 letters and refers to a lung disease caused by the inhalation of very fine silica particles, such as those found in volcanic ash.

However, it's worth noting that there are other words that are longer than this if you include scientific or technical terms, chemical names, or coined words. For example, the chemical name for the protein known as titin contains 189,819 letters, making it one of the longest words ever used. However, this word is not commonly used in everyday language.