From giga- +‎ year.

gigayear (plural gigayears)

1. A billion years, a thousand thousand thousand years.

   Synonym: giga-annum

   • 1967, Rankama, Kalervo, Institute of Geology and Mineralogy, University of Helsinki, Helsinki, Finland, Megayear and Gigayear: Two Units of Geological Time:
   • 1982, A. M. Clarke, “Carboniferous Sedimentary Basins of Northern Europe and the Nature of Emergence around the Margins of the Mesozoic Rifted Sedimentary Basin of the North Sea”, in Philosophical Transactions of the Royal Society of London, 305(1489), pp207-208:

     These syntheses gave a fresh geological, as opposed to solely geophysical, view of the up to multi-megayear and near gigayear mechanism responsble for the palimpsests of a general process of change in the upper crust: a process that we may observe, often apparently 'frozen' -- because it is so slow, at different steps in the process, in different places within the upper crust.

   • 1993, Rosemary F.G. Wyse, “The physical interpretation of morphology”, in J. Michael Shull, Harley A. Thronson, Jr., editors, The Environment and Evolution of Galaxies, page 308:

     The illustrative example here provides for a few gigayear spread in ages of tracers of the stellar halo in a spiral (this is obviously sensitive to the chosen turnaround redshift) as found for the Milky Way field stars (Schuster & Nissen 1989) and globular clusters (reviewed by Bolte 1993).

   • 2002, William Harris of McMaster University in Hamilton, Canada. (quoted on Astronomy.com)

     It's hard to tell the difference between a 5-gigayear, 8-gigayear, and 10-gigayear cluster for such a sparse object as this unless you've got very high-quality photometry.

   • 2007, Linda Siobhan Sparke, John S. Gallagher, III, Galaxies in the Universe, page 343:

     The fraction could be as high as 100% if the nuclear activity lasts much less than a gigayear.

• (abbreviation) Gyr

• kiloyear
• megayear