As is illustrated by the first answer, scientific names of considerable length can be generated, but that seems almost artificial.

And many words that have at one time or another been hailed as “the longest word” in English are not actually USED; some have even been manufactured just to MAKE them the “longest word” (Compare what the folks from the Oxford English Dictionary say in the link below –and especially their observation that “the formal names of chemical compounds are almost unlimited in length”.)

My vote would be for the longest PLACE name in America — the name of a lake which local residents actually DO like to call by this 45-letter name —

Chargoggagoggmanchauggagoggchaubunagungamaugg

an Indian name meaning, “”Englishmen at Manchaug at the Fishing Place at the Boundary” (actually an expansion on the lake’s original and official name of “Chaubunagungamaugg”)

http://www.oldewebster.com/history/lake_chargogg.htm

This also happens to make it just as long as the longest word listed in the Oxford English Dictionary –one they themselves acknowledge was manufactured (though I think it THEN came to be used). Anyway, if we disallow place names, this would then have to be the “longest English word in the dictionary”

pneumonoultramicroscopicsilicovolcanoconiosis

http://www.askoxford.com/asktheexperts/faq/aboutwords/longestword?view=uk

(Check that link for other contestants.)

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(I must admit I was disappointed to discover the REAL meaning of the lake name. I always liked the popular explanation You Fish on Your Side, I Fish on My Side, Nobody Fish in the Middle!”)

acetylseryltyrosylserylisoleucylthreonylserylprolylserylglutaminyl-

phenylalanylvalylphenylalanylleucylserylserylvalyltryptophylalanyl-

aspartylprolylisoleucylglutamylleucylleucylasparaginylvalylcysteinyl-

threonylserylserylleucylglycylasparaginylglutaminylphenylalanyl-

glutaminylthreonylglutaminylglutaminylalanylarginylthreonylthreonyl-

glutaminylvalylglutaminylglutaminylphenylalanylserylglutaminylvalyl-

tryptophyllysylprolylphenylalanylprolylglutaminylserylthreonylvalyl-

arginylphenylalanylprolylglycylaspartylvalyltyrosyllysylvalyltyrosyl-

arginyltyrosylasparaginylalanylvalylleucylaspartylprolylleucylisoleucyl-

threonylalanylleucylleucylglycylthreonylphenylalanylaspartylthreonyl-

arginylasparaginylarginylisoleucylisoleucylglutamylvalylglutamyl-

asparaginylglutaminylglutaminylserylprolylthreonylthreonylalanylglutamyl-

threonylleucylaspartylalanylthreonylarginylarginylvalylaspartylaspartyl-

alanylthreonylvalylalanylisoleucylarginylserylalanylasparaginylisoleucyl-

asparaginylleucylvalylasparaginylglutamylleucylvalylarginylglycyl-

threonylglycylleucyltyrosylasparaginylglutaminylasparaginylthreonyl-

phenylalanylglutamylserylmethionylserylglycylleucylvalyltryptophyl-

threonylserylalanylprolylalanylserine

I would say that it is the name Erica