eXogenous

Exogenous is an adjective used across disciplines to describe something that originates from outside a given system, organism, or context. It contrasts with endogenous, which refers to factors or processes produced within the system. The term comes from Latin, roughly meaning “originating outside.”

In biology and medicine, exogenous factors are external to the body or organism. This includes pathogens, toxins, and drugs administered from outside, as well as environmental influences like nutrition and sunlight. Exogenous substances are those introduced externally, while endogenous substances are generated by the body's own metabolism. Exogenous cues or stimuli, such as light-dark cycles, can influence physiological processes like circadian rhythms.

In economics and the social sciences, exogenous variables or shocks originate outside the model or system. An exogenous shock is an external event—such as policy changes, natural disasters, or geopolitical upheavals—that affects outcomes but is not explained by the system’s internal dynamics. Statistically, exogenous variables are those not determined by the model and uncorrelated with the error term, in contrast to endogenous variables that are determined within the model.

In geology, exogenous processes are external forces like weathering and erosion driven by the environment, whereas endogenous processes involve internal forces such as tectonics. The term also appears in linguistics and philosophy to denote substances, influences, or origins external to a given framework.

Overall, exogenous denotes external origin or input relative to a specified system or context.