geologic age . (a) The age of a fossil organism or of a particular geologic event or feature referred to the geologic time scale and expressed in terms either of years or centuries ( absolute age ) or of comparison with the immediate surroundings ( relative age ); an age datable by geologic methods. (b) The term is also used to emphasize the long-past periods of time in geologic history, as distinct from present-day or historic times. Cf: age [geochron].