Related Clues:
- One billion years, in astronomy
- Group of eras
- Series of periods
- Vast time period
- Geologic division
- Longest division of geological time
- Forever, figuratively
- Time measured in ages
- Geologist's division
- Astronomical unit
- Countless centuries
- Awfully long time
- Geologic period
- Unbelievably long time
- Mighty long time: Var.