Exogenous variables in causal modeling are the variables with no causal links (arrows) leading to them from other variables in the model. In other words, exogenous variables have no explicit causes within the model.
The concept of exogenous variable is fundamental in path analysis and structural equation modeling.
The complementary concept is endogenous variable
Note: classification of a particular variable as exogenous depends on the chosen causal model: the same variable may be exogenous in one model and endogenous in another model based on exactly the same set of variables.