Titin: derived from the Greek titan (a giant deity, anything of great size). This is a fitting name considering its name. Because it is the largest known protein it also has the longest IUPAC name which starts with “methiony...” and ends with “...isoleucine” and contains about 1.5 thousand letters in between.
This makes it the longest word in not only the English language but also in any other language on Earth. Titin is very important in the contraction of muscle tissue.
Only a few lexicographers accept chemical formulae as words, even if the chemical makes it possible for them to write words. As a result you won’t find the word in most dictionaries. But hey, Google doesn’t seem to mind it, and so it’s still considered by many to be the longest word.