世（epoch）是指地质学和考古学的一个时间单位。

最初由莱伊尔在他的《地质学原理》一书中，首先使用「世」的附加时间单元来表示恐龙时代以后的时期。其中由远而近有渐新世（Oligocene「有一点近」）、中新世（Miocene「颇近」）、上新世（Pliocene「较近」）、更新世（Pleistocene「最近」）等。

莱伊尔原本想用「-synchronous」作为这几个字的后缀，却遭当时具有影响力的惠卫耳（William Whewell，1794年－1866年）牧师基于字根学上的理由反对，建议改用「-eous」当后缀，最后的「-cene」算是折衷的办法。

In geochronology, an epoch is a subdivision of the geologic timescale that is longer than an age and shorter than a period. We are currently living in the Holocene Epoch of the Quaternary Period. Rock layers deposited during an epoch are called a series. Series are subdivisions of the stratigraphic column that, like epochs, are subdivisions of the geologic timescale. Like other geochronological divisions, epochs are normally separated by significant changes in the rock layers to which they correspond.