Actually, it is reputed to be the longest word in any language, and not just English. It is the technical name for titin, which is a human protein that helps with the elasticity of muscles.
Here is Dmitry Golubovskiy (CEO of Esquire Russia), wasting 3 1/2 hours of his life pronouncing it. (Please excuse his heavy accent.)
A file with the word itself, spelled out in all its glory, can be found here.