1. n. [Geology]

Subdivisions of sequences that consist of discrete depositional units that differ in geometry from other systems tracts and have distinct boundaries on seismic data. Different systems tracts are considered to represent different phases of eustatic changes. A lowstand systems tract develops during times of relatively low sea level; a highstand systems tract at times of high sea level; and a transgressive systems tract at times of changing sea level.

See: eustasy,  regression,  sequence,  sequence stratigraphy,  transgression