Untold centuries is a phrase that refers to a very long period of time, usually one that is impossible to measure. The term can be broken down into three separate words: AEONS (5 letters), EONS (4 letters), and EON (3 letters). AEONS is the longest word and refers to an indefinite and immeasurable amount of time, while EONS is a slightly shorter period of time and EON is the shortest, referring to a single unit of time.