In linguistics, that question is answered by another question: how do you define “word”? A word could be a proper noun (name of a person or a place name), a technical or scientific term, an invented term (usually nonsensical or frivolous — think Mary Poppins and the word that rhymes with “atrocious”), lengthy constuctions (adding prefixes and suffixes to a word) or a compound word (combining several words, for example great-great-grandfather). If we exclude all these (my apologies to all avid logophiles) we arrive at a more sensible question that can be reframed and answered. In other words, what is the longest word we are likely to come across in general reading and writing? If you do not suffer from hippopotomonstrosesquipedaliophobia (the fear of long words), then please read on.
The longest word in:
Common English writing: inappropriateness (17), incomprehensibility (19), uncharacteristically (20), deinstitutionalization (22), counterrevolutionaries (22)
Less common are the words disproportionableness, incomprehensibilities (both 21)
Old Testament: evilfavouredness, lovingkindness (both 16)
New Testament: covenantbreakers, unprofitableness (both 16)
Works of Shakespeare: anthropophaginian, indistinguishable, undistinguishable (all three 17); incomprehensible (16)
This list excludes Shakespeare’s use of the latin term, honorificabilitudinitas, 27 letters, that appears in Act V, Scene 1 of Love’s Labour’s Lost.
Works of Dickens: incomprehensibility (19) that appears in Little Dorrit and David Copperfield.
The longest word found in dictionaries:
Oxford English Dictionary:
antidisestablishmentarianism (28), floccinaucinihilipilification (29),
pseudopseudohypoparathyroidism (30)
pneumonoultramicroscopicsilicovolcanoconiosis (45)
Random House Dictionary and Webster’s Third New International Dictionary, Unabridged: pneumonoultramicroscopicsilicovolcanoconiosis (45)
Merriam-Webster’s Collegiate Dictionary: electroencephalographically,
ethylenediaminetetraacetate (27)
The longest word that contains all vowels (and sometimes y):
Facetiously
The longest word that does not contain any vowels:
Rhythms
Read related posts: What is the Longest Word in English Language?
What is the Longest English Word Without Repeated Letters?
Words with Letters in Alphabetical Order
Word Oddities: Fun with Vowels
What is an Abecedarian Insult?
Difficult Tongue Twisters
Rare Anatomy Words
What Rhymes with Orange?
Sources: http://www.artbible.info/concordance; oxforddictionaries.com; http://www.merriam-webster.com/help/faq/long_words.htm, http://www.fun-with-words.com/word_longest.html; http://www.dictionary.reference.com; http://answers.yahoo.com/question/index?qid=1006052431981
http://victorian.lang.nagoya-u.ac.jp/concordance/dickens/
Making the Alphabet Dance by Ross Eckler St. Martin’s Press (1996); Crazy English by Richard Lederer, Pocket Books (1989)
Atkins Bookshelf 