Is "antidisestablishmentarianism" the longest word in English?
Although antidisestablishmentarianism is an interesting word, it definitely is NOT the longest one in English.
Antidisestablishmentarianism, with 28 letters, is defined in The Oxford English Dictionary (a 20-volume historical dictionary of English) as follows: "Properly, opposition to the disestablishment of the Church of England (rare): but popularly cited as an example of a long word." Merriam-Webster doesn't enter antidisestablishmentarianism in any of its dictionaries because the evidence indicates that the word is almost never used anymore.
The longest word that you'll find in our Webster's Third New International Dictionary, Unabridged, is pneumonoultramicro-scopicsilicovolcanoconiosis, which has 45 letters and which is a disease of the lungs caused by inhaling very fine irritant particles. Miners are especially subject to this disease. However, most use of the word is simply in reference to how long it is. Another long word (27 letters) that you'll find in the Merriam-Webster's Online Dictionary, is electroencephalographically.
Floccinaucinihilipilification at 29 letters has also been claimed to be the longest word in English. While it is entered in the Oxford English Dictionary with the meaning "the action or habit of estimating as worthless," it is not in any of the Merriam-Webster dictionaries because our evidence shows us that it is, again, almost always used simply as an example of a long word.
Honorificabilitudini-tatibus has 27 letters. This Latin word meaning "with honor" was deployed by Shakespeare in Love's Labor's Lost, when Costard says, "I marvel thy master hath not eaten thee for a word; for thou art not so long by the head as honorificabilitudini-tatibus: thou are easier swallowed than a flap-dragon."
As an over-the-top bonus, here's one very, very long name mentioned by Paul Dickson in What's in a Name? (Merriam-Webster, 1996, out of print). In his chapter on "Record-Breaking Names," Dickson mentions one, a man found on the island of Vanua Levu in 1967 who has or had 120 letters in his name, including, you will find, the name of a major appliance manufacturer (Amana): Tuimidrenicagitokalauenatobakonatewaenanonaskoktamanakisomosomome-lakivolainaivolanikawabulasokomailesutalesigadua.