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.