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The longest technical word in English is the scientific name for the protein titin, at 189,819 letters. Titin is the largest known polypeptide in the human body, composed of 34,350 amino acids. Though lexicographers regard generic names of chemical compounds as verbal formulae rather than English words, for its sheer length it is often included in longest-word lists.

The 45-letter word pneumonoultramicroscopicsilicovolcanoconiosis is the longest English word that appears in a major dictionary. Originally coined to become a candidate for the longest word in English, the term eventually developed some independent use in medicine. It is referred to as "P45" by researchers.

Supercalifragilisticexpialidocious, at 34 letters, is a long English word that became famous after inclusion in a song sung by Julie Andrews and Dick Van Dyke in the Mary Poppins 1964 film.

Antidisestablishmentarianism, at 28 letters, is the longest non-coined, non-technical English word. It refers to a 19th century political movement that opposed the disestablishment of the Church of England as the state church of England.

Floccinaucinihilipilification, at 29 letters (and meaning the act of estimating something as being worth so little as to be practically valueless, or the habit of doing so), is the longest non-technical, coined word in the English language.