For most people, an eon is just a non-specific ‘long period of time’, and that’s the way it’s been for eons and eons.
Definition of eon/aeon
1: an immeasurably or indefinitely long period of time: AGE…
Scientists, however, like to be very precise about things so they adopted the name eon to describe a specific unit of time.
Eon, Long span of geologic time. In formal usage, eons are the longest portions of geologic time (eras are the second-longest). … Less formally, eon often refers to a span of one billion years.
Encyclopedia Britannica further explains that “Three eons are recognized: the Phanerozoic Eon (dating from the present back to the beginning of the Cambrian Period), the Proterozoic Eon, and the Archean Eon.” Another Britannica article explains that there is another called the Hadean Eon, so I guess there are 4.
Eon has a number of meanings. In Astronomy, an eon refers to 1 billion (10*9) years. But it also refers to a very long, unspecified period of time, or specific geologic stages of the Earth.
That the answer of eon = 10*9 (10 to the 9th degree) years is the most appropriate, however it is not the most common.
The term eon (or aeon) is frequently used as a term for a very long but un-specified time period, much the same way “forever” and “ages” are used.
In Earth history, stages of the earth’s development are divided into four major eons (Hadean, Archean, Proterozoic and Phanerozoic), which are in turn divided into eras and periods. These geologic eons are distinguished by major stages in Earth’s geologic history, rather than fixed periods of time.