Series is a term defining a unit of rock layers formed during a certain interval of time (a chronostratigraphic unit); it is equivalent (but not synonymous) to the term geological epoch (see epoch criteria) which defines the interval of time itself, although the two words are sometimes confused in informal literature.
The geological timescale has all systems in the Phanerozoic eonothem subdivided into series.
Some of these have their own names; in other cases a system is simply divided into a Lower, Middle and Upper series, with official series being capitalized and unofficial designations (such as "middle Cretaceous") being left uncapitalized.
Currently series are limited to the Phanerozoic, but the ICS has stated its intention of subdividing the three systems of the Neoproterozoic (Ediacaran, Cryogenian and Tonian) into stages too.
Systems can include many lithostratigraphic units (for example formations, beds, members, etc.)