Societal collapse usually refers to the fall or disintegration of human societies often along with their life support systems.
Societal collapse broadly includes both quite abrupt societal failures typified by collapses (such as that of the Mayan Civilization), as well as more extended gradual declines of superpowers (like the Roman empire in Western Europe).
The general subject arises in anthropology, history, sociology, politics and other fields, and more recently in complex systems science.
What distinguishes the more dramatic failures of human societies, seeming to deserve the term "collapse," from less dramatic long term decline is not widely agreed on.
The subject may include any other long term decline of a culture, its civil institutions or other major characteristics of it as a society or a civilization.