A period of incalculable duration: Age.
This has been the case for eons and eons: the great majority of people use the term “eon'' to refer to a nebulous "long period” dating at least as far back as 1642. However, scientists want precision, so they adopted the term "eon" to denote a specific unit of time.
Eon, a vast geologic epoch. Eons are the longest geological time units in official use (eras are the second-longest) … A period of one billion years is informally referred to as an eon.
According to Encyclopedia Britannica, the Phanerozoic Eon (stretching from the present to the beginning of the Cambrian Period), the Proterozoic Eon, and the Archean Eon are the three recognized eons. According to a separate Britannica article, there are four, with the Hadean Eon being one of them.
Following is a list of geologic time units in decreasing order of duration.
Geologic era chronologies denote the periods between geomagnetic reversals.