2. The period of time that extends from the beginning of the world to the present day.
3. An interval of time occupied by the earth's geologic history, extending from about 3.9 billion years ago (corresponding to the age of the oldest known rocks) to the present day.
In other words, the part of the earth's history that is recorded in rock strata.
The geological time scale is classified in intervals distinguished by characteristic geological and biological features; as indicated by the following longest to the shortest durations: eon (one thousand million years), era (divided into several periods), period (unit of geological time during which a system of rocks is formed), and epoch (geologic time that is a subdivision of a period).
Since new rocks are generally deposited on top of existing material, those lower down are oldest.
The strata of rock are classified according to their age, and a time scale corresponding to this can be constructed.
The main divisions (eras) are the Paleozoid, Mesozoic, and Cenozoic. These are further subdivided into periods and epochs.