45) PNEUMONO­ULTRA­MICRO­SCOPIC­SILICO­VOLCANO­CONIOSIS (also spelled PNEUMONO­ULTRA­MICRO­SCOPIC­SILICO­VOLCANO­KONIOSIS) = a lung disease caused by breathing in particles of siliceous volcanic dust.
This is the longest word in any English dictionary. However, it was coined by Everett Smith, the President of The National Puzzlers' League, in 1935 purely for the purpose of inventing a new "longest word". The Oxford English Dictionary described the word as factitious. Nevertheless it also appears in the Webster's, Random House, and Chambers dictionaries.
(37) HEPATICO­CHOLANGIO­CHOLECYST­ENTERO­STOMIES = a surgical creation of a connection between the gall bladder and a hepatic duct and between the intestine and the gall bladder.
This is the longest word in Gould's Medical Dictionary.

(34) SUPER­CALI­FRAGI­LISTIC­EXPI­ALI­DOCIOUS = song title from the Walt Disney movie Mary Poppins.
It is in the Oxford English Dictionary.

"But then one day I learned a word
That saved me achin' nose,
The biggest word you ever 'eard,
And this is 'ow it goes:
Supercalifragilisticexpialidocious!"
(30) HIPPOPOTO­MONSTRO­SESQUIPED­AL­IAN = pertaining to a very long word.
From Mrs. Byrne's Dictionary of Unusual, Obscure and Preposterous Words.

(29) FLOCCI­NAUCINI­HILIPIL­IFICATION = an estimation of something as worthless.
This is the longest word in the first edition of the Oxford English Dictionary. Interestingly the most common letter in English, E, does not appear in this word at all, whilst I occurs a total of nine times. The word dates back to 1741. The 1992 Guinness Book of World Records calls flocci­nauci­nihili­pilification the longest real word in the Oxford English Dictionary, and refers to pneumono­ultra­micro­scopic­silico­volcano­koniosis as the longest made-up one.

(28) ANTI­DIS­ESTABLISH­MENT­ARIAN­ISM = the belief which opposes removing the tie between church and state.
Probably the most popular of the "longest words" in recent decades.

(27) HONORI­FICABILI­TUDINI­TATIBUS = honorableness.
The word first appeared in English in 1599, and in 1721 was listed by Bailey's Dictionary as the longest word in English. It was used by Shakespeare in Love's Labor's Lost (Costard; Act V, Scene I):

"O, they have lived long on the alms-basket of words.
I marvel thy master hath not eaten thee for a word; for thou art not so long by the head as honorificabilitudinitatibus: thou art easier swallowed than a flap-dragon."
Shakespeare does not use any other words over 17 letters in length.

(27) ELECTRO­ENCEPHALO­GRAPHICALLY
The longest unhyphenated word in Merriam-Webster's Collegiate Dictionary (10th Ed.), joint with ethylene­diamine­tetraacetate (see below).

(27) ANTI­TRANSUB­STAN­TIA­TION­ALIST = one who doubts that consecrated bread and wine actually change into the body and blood of Christ.

(21) DIS­PRO­PORTION­ABLE­NESS and (21) IN­COM­PREHEN­SIB­ILITIES
These are described by the 1992 Guinness Book of World Records as the longest words in common usage.

Some say SMILES is the longest word because there is a MILE between the first and last letters!

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Chemical Terms
Two chemical terms (3,641 and 1,913 letters long) have appeared in the Guinness Book of World Records. They were withdrawn because they have never been used by chemists, and there is no theoretical limit to the length of possible legitimate chemical terms. A DNA molecule could have a name of over 1,000,000,000 letters if it was written out in full.
(1,185)