What is the longest word in the world and what does it mean?

Titin is the largest protein yet discovered. Its full chemical name is 189,819 letters long and, depending on how you define a "word" is the longest word in the English language. It is not included in the dictionary - few lexicographers accept chemical formulae as words. But when people go round Googling "What's the longest word in the English language?", they don't really care if it's a "real" word or not.