Definition of "gigayear"

1967, Rankama, Kalervo, Institute of Geology and Mineralogy, University of Helsinki, Helsinki, Finland, Megayear and Gigayear: Two Units of Geological Time

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.

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

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).

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

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.

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

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

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