gigayear

A gigayear, a billion years, is a common unit for cosmological times and time intervals, convenient, given that the current age-determination of the universe is between 13 and 14 gigayears. Abbreviations include Gy and Gyr. The word eon (or aeon) has sometimes been used within astronomy to indicate a gigayear, but is more widely used to mean "a very long time", and in geology it is used for specific periods within the geological timeline. A gigayear is on the order of 10¹⁶ seconds, more specifically 3.15576 × 10¹⁶.

Given the finite speed of light (c), when we observe something at some number of billions of light-years' distant, we are observing it that many gigayears in its past.

The term giga-anum (Gya) is used specifically for a billion years in the past, i.e., counting backward from now.