The Ages of the Earth

Based on the dating of rocks found in the Earth’s crust, the Earth is about four billion years old.

When dating the earth, time periods used are eons, eras, epochs and so on.

What does it all mean?

Eon: the largest division of geologic time, comprising two or more eras. (When talking about eons with family and friends, make sure you know the discipline of the person you’re talking to. The above is the geologic definition, referring to the earth. Of course things are never that simple….if you’re talking in terms of astronomy, it means “one billion years”.

Era: a period of time marked by distinctive character or events, etc. a major division of geologic time composed of a number of periods.

Period: a rather large interval of time that is meaningful in the life of a person, in history, etc., because of its particular characteristics. The period is the basic unit of geologic time, during which a standard rock system is formed: comprising two or more epochs and included with other periods in an era.

Epoch: a particular period of time marked by distinctive features, events, etc. any of several divisions of a geologic period during which a geologic series is formed. A unit of geologic time that is a division of a period.
(Again, epoch has a different meaning in astronomy than in geology).

Age: a particular period of history, as distinguished from others; a historical epoch.
a. a period of the history of the earth distinguished by some special feature: the Ice Age.
b. a unit of geological time, shorter than an epoch, during which the rocks comprising a stage were formed.

Stage: a division of stratified rocks corresponding to a single geologic age.

And there you have it!

Word Origins – from the Online Eytmology Dictionary

eon
1640s, from L. aeon, from Gk. aion "age, vital force, lifetime," from PIE base *aiw- "vital force, life, long life, eternity" (cf. Skt. ayu "life," Avestan ayu "age," L. aevum "space of time, eternity," Goth. aiws "age, eternity," O.N. ævi "lifetime," Ger. ewig "everlasting," O.E. a "ever, always").

era
1615, from L.L. æra, era "an era or epoch from which time is reckoned," probably identical with L. æra "counters used for calculation," pl. of æs (gen. æris) "brass, money" (see ore). The L. word's use in chronology said to have begun in 5c. Spain (where, for some reason unknown to historians, the local era began 38 B.C.E.; some say it was because of a tax levied that year). Like epoch, in Eng. it originally meant "the starting point of an age;" meaning "system of chronological notation" is c.1646; that of "historical period" is 1741.

period
1413, "course or extent of time," from M.L. periodus "recurring portion, cycle," from L. periodus "a complete sentence," also "cycle of the Greek games," from Gk. periodos "rounded sentence, cycle, circuit, period of time," lit. "going around," from peri- "around" + hodos "a going, way, journey" (see cede). Sense of "repeated cycle of events" led to that of "interval of time."

epoch
1610s, "point marking the start of a new period in time" (e.g. the founding of Rome, the birth of Christ, the Hegira), from M.L. epocha, from Gk. epokhe "stoppage, fixed point of time," from epekhein "to pause, take up a position," from epi- "on" + ekhein "to hold." Transferred sense of "a period of time" is 1620s; geological usage (not a precise measurement) is from 1802.

age
c.1300, "long but indefinite period in human history," from O.Fr. aage, from V.L. *aetaticum (cf. Sp. edad, It. eta, Port. idade "age"), from L. aetatem (nom. aetas), "period of life," from aevum "lifetime, eternity, age," from PIE base *aiw- "vital force, life, long life, eternity" (see eon). Meaning "time something has lived, particular length or stage of life" is from early 14c.