Despite calims put forward for other words, the longest word in most English dictionaries is still "antidisestablismentarianism". It has twenty-eight letters and refers to the doctrine that an established church should maintain the status as the official church of a country. The longest word in the Oxford English Dictionary is "floccinaucinihilipilification", twenty-nine letters.
However, the Guiness Book of Records states that the protein tryophan synthetase has a full name which requires 1913 letters. The word "supercalifragilisticexpialidocious", thirty-four letters does not exist at all and was invented for use in Mary Poppins (which probably means that it does exist now). The comedian Red Skelton is credited with discovering the world's longest word. He said it was the one that follows, "And now a word from our sponsors".