Many Millennia is a term used to describe a very long period of time. It is often used to refer to a period of time that is much longer than a millennium, which is usually defined as 1, 000 years. The term 'EON' is a 3-letter word that is used to refer to an indefinite and very long period of time, while 'EONS' is a 4-letter word that is used to refer to multiple EONS. Therefore, the crossword solutions for the term 'Many Millennia' are EON and EONS.