I'm assuming there isn't a word that contains every letter in the alphabet, so which word contains the most?
antidisestablishmentarianism - 12
psychotherapy - 12
handcraftsmanship - 13
Among words in a certain word list I found:
It has ten words with 16, including:
There are several dozen with 15, including:
and several hundred with 14 (including a few 14-letter words like 'pseudomythical', 'hydropneumatic' and 'ambidextrously').
In case anyone cares, this was done with Python, something like:
A better word list (one that includes subdermatoglyphic, for instance!) may give longer words. And you didn't ask about two words, but just for reference: this says "Ralph Beaman proposes formaldehydesulphoxylic ventriloquizing as a two-word combination having 22 different letters which should be difficult to beat"; I haven't checked this.
Uncopyrightable is supposedly the longest word in the English language with no repeated letters, which makes it a logical starting point. That sets the bar at 15 unique letters.
Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch clocks in at 18 distinct characters, but arguably a near-infinite number of 25-character answers exist, since organic chemistry nomenclature allows one to build valid hypothetical compound names of essentially arbitrary length, and the full alphabet less only J appears on the periodic table (though this does require one to refer to rutherfordium by its old placeholder name unnilquadium or arrogate a placeholder name for a transuranic element with an atomic number ending in 4 that hasn't been named yet, so it may only get us to 24 characters).
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