The term 'countless years' can be represented by three different crossword solutions: EON (3 letters), AGES (4 letters), and EONS (4 letters). EON is a very long period of time, usually millions or billions of years. AGES is a period of time that is usually associated with a particular event or era. EONS are even longer than EONS, and are typically used to describe the length of time since the Earth was formed. All three terms refer to an indefinite amount of time, making them suitable for describing the phrase 'countless years'.