This is a simple question.
The answer is not so simple. I made a short research on the Internet and here is what I found.
Let’s start with a little joke:
The ‘SMILES’ -is the really longest English word.
Really, there is a MILE between two Ss: SMILES!
The longest English word that can be using only vowels:
The longest English word which has all letters in the alphabetical order:
The longest English word that can be constructed with only one vowel:
The longest English word which you can type only with the left hand:
The longest English word which is typable by alternating left and right hands:
The longest English word with no repeated letters:
The longest dune name in English:
The longest English word in Shakespeares’s works:
The longest hypothetically legal Scrabble English word:
The longest unchallenged nontechnical English word:
The longest non-coined and nontechnical English word:
The longest English word in a major dictionary:
The longest English lake name:
The longest English hill name:
The longest English place name:
The longest English word coined by a major author:
The longest English Chemical name:
Note: The longest word cited by the Guinness Book of Records, which allegedly represents the name for human mitochondrial DNA, is more than 207,000 letters long. Some types of DNA could have as many as a billion letters if fully written out.