A cyclothem is a repeating sequence of sedimentary rock layers that represents alternating marine and non-marine conditions, typically associated with the cyclic rise and fall of sea levels. Cyclothems are important in the study of stratigraphy and sedimentology, particularly in understanding the depositional environments of coal-bearing sequences.
Reference: Heckel, P. H. (1986). “Sea-Level Curve for Pennsylvanian Eustatic Marine Transgressive-Regressive Depositional Cycles along Midcontinent Outcrop Belt, North America.” Geology, 14(4), 330-334.