The answer to this question has been up for debate for some time. One of the reasons the English language is so rich and so flexible is that, by combining different roots and affixes (prefixes and suffixes), anyone can create a new word whose meaning can be understood by examining its various parts.
So, really, it depends on how you want to define word. Does it need to have appeared in a dictionary? Does it need to have appeared in more than one piece of literature? Does it need to be in standard usage?
Here are some contenders for the title of Longest English Word. You decide which one wins:
- Titin: Titin is a protein found in striated muscle. Proteins derive their chemical names from the amino acids that make them up. Titin, whose chemical formula is C132983H211861N36149O40883S693, is the largest known protein, consisting of 34,350 amino acids. Its chemical name (which is never actually used anywhere) is 189,819 letters long.
- Pneumonoultramicroscopicsilicovolcanoconiosis: This 45-letter doozie is a disease caused by the long-term inhalation of silica dust, a long-time danger to coal miners. Though it was coined in 1935 by the president of the National Puzzlers' League to describe what was normally called silicosis, it has managed to appear in a few dictionaries.
- Pseudopseudohypoparathyroidism: Another medical term, this 30-letter whopper, usually abbreviated as pseudoPHP, wasn't a coined expression and is actually used in medical writing and diagnoses.
- Floccinaucinihilipilification: Defined as "the act or habit of estimating as worthless" by the Oxford English Dictionary, this 29-letter mouthful is perhaps the longest non-technical word to appear in a dictionary. It has been used a handful of times in British literature, going back to the mid-18th century.
- Antidisestablishmentarianism: A favorite among long-word enthusiasts, at 28 letters, this is likely the longest English word to have been in common usage. You can find out how this word came to be here. Although this is the form you'll find most often, through the magic of English suffixes, you can transform it into antidisestablishmentarianistically, a 34-letter word that edges out the two previous contenders.
If you enjoy lengthy language, one word you should add to your vocabulary is sesquipedalian. This useful word literally means "a foot and a half long" and is used to describe a loquacious elocutionist's inordinately gargantuan verbiage. That is, the unnecessarily long words that a speaker uses.