1. The formal geochronologic unit, longer than a geologic age and shorter than a geologic period, during which the rocks of the corresponding series were formed (from Glossary of Geology 1997). 2. A term used informally to designate a length (usually short) of geological time, for example, glacial epoch. 3. In paleomagnetism, a date to which measurements of a time-varying quantity are referred, for example, “a chart of magnetic declination for epoch 1965. 0,” or informally as a magnetic polarity epoch. Magnetic polarity refers to whether the geomagnetic field was like it is today (normal polarity) or whether the north and south poles are reversed from their present configuration (reversed polarity).