The longest scientific term in English is the full chemical name of the world’s largest known protein, titin. Beginning with Methionyl… and ending with …isoleucine, the word contains 189,819 letters.
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.
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.
In “How to Build a Better Vocabulary”, there is also a word attributed to P. T. Barnum, who wanted to have the record for inventing the longest word, and apparently learned just enough chemistry to coin: Di-isobutyl-phenoxy-ethoxy-ethyl-dimethyl-benzyl-ammonium-chloride, where the hyphens normally are not present but show how the word is built up. It also begins with the unusual start of “DII..”. Here it is, all 58 letters long: diisobutylphenoxyethoxyethyldimethylbenzylammoniumchloride.
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.