From Wikipedia:
The English word antidisestablishmentarianism (UK /ˌæntidɪsɪˌstæblɪʃmənˈtɛəriənɪzəm/ ⓘ US /ˌæntaɪ-/ ⓘ) is notable for its unusual length of 28 letters and 12 syllables, and is one of the longest words in the English language.[1] It has been cited as the longest word in the English language (excluding coined and technical terms), although some dictionaries do not recognize it because of its low usage in everyday lexicon.[2]
Antidisestablishmentarianism is a political position that originated in 19th century Britain. The position opposed proposals at that time to remove the Anglican Church‘s status as the established church of England, Ireland, and Wales, but not in Scotland, which had and still has its own separate national church.[3]
The word became known in the public realm in the US via a popular television game show in the 1950s, The $64,000 Question, when a young contestant correctly spelled it to win.[4]
There is varied recognition of antidisestablishmentarianism among major English dictionaries. Merriam-Webster does not recognize the word because it is practically unused in the modern era, although they do include disestablishmentarianism and antiestablishmentarianism.[5][6]
The The American Heritage Dictionary of the English Language,[7] Chambers Dictionary,[8] and Oxford English Dictionary[9] similarly exclude antidisestablishmentarianism, but keep smaller variations.
Dictionaries that do include the word are the Cambridge Advanced Learner’s Dictionary,[citation needed] Collins English,[10] and Dictionary.com.[11]
The longest word found in a major dictionary is pneumonoultramicroscopicsilicovolcanoconiosis, but this is a technical term that was coined specifically to be the longest word.[1][12]
Pneumonoultramicroscopicsilicovolcanoconiosis, from Wikipedia:
Pneumonoultramicroscopicsilicovolcanoconiosis (/ˌnjuːmənoʊˌʌltrəˌmaɪkrəˈskɒpɪkˌsɪlɪkoʊvɒlˌkeɪnoʊˌkoʊniˈoʊsɪs/ ⓘ[1][2]) is a 45-letter word coined in 1935 by Everett M. Smith, the then-president of the National Puzzlers’ League. It has sometimes been used as a synonym for the occupational disease known as silicosis, but it should not be, as most silicosis is not related to mining of volcanic dusts. Silicosis is a form of occupational lung disease caused by inhalation of crystalline silica dust, and is marked by inflammation and scarring in the form of nodular lesions in the upper lobes of the lungs. It is a type of pneumoconiosis. Pneumonoultramicroscopicsilicovolcanoconiosis is the longest word in the English language published in a popular dictionary, Oxford Dictionaries, which defines it as “an artificial long word said to mean a lung disease caused by inhaling very fine ash and sand dust”.[3]
Clinical and toxicological research conducted on volcanic crystalline silica has found little to no evidence of its ability to cause silicosis/pneumoconiosis-like diseases and geochemical analyses have shown that there are inherent factors in the crystalline structure which may render volcanic crystalline silica much less pathogenic than some other forms of crystalline silica.[4][5]
