age [geochron] . (a) The formal geochronologic unit of lowest rank, below epoch [geochron] , during which the rocks of the corresponding stage or substage (chronostratigraphic units) were formed. See also: subage . (b) A term used informally to designate a length of geologic time during which the rocks of any stratigraphic unit were formed. (c) A division of time of unspecified duration in the history of the Earth, characterized by a dominant or important type of life form; e.g. the "age of mammals". (d) The time during which a particular geologic event or series of events occurred or was marked by special physical conditions; e.g. the "Ice Age". (e) The position of anything in the geologic time scale; e.g. "the rocks of Miocene age". It is often expressed in years. Cf: geologic age.