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.
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).
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.
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