The term 'very long time period' can be summarized by the crossword solution 'EON', which is a three-letter word. An EON is an immeasurably long period of time, typically used to describe the longest divisions of geological time. It is usually divided into two eras, the Precambrian and the Phanerozoic. The Precambrian era is the longest, spanning from 4.6 billion years ago to 541 million years ago. The Phanerozoic era is the most recent, beginning 541 million years ago and continuing to the present day.