The term 'Immeasurable Periods' can be summed up by the crossword solution 'EONS', which is a four-letter word. EONS are extremely long periods of time, usually measured in billions of years. They are used to describe the age of the universe and the length of time that certain geological processes have been occurring. EONS are also used to describe the immense amount of time that has passed since certain events occurred, such as the extinction of dinosaurs.