A model suffers a concept drift if the very meaning of the modeling problem changes: for example, the criteria to diagnose an illness change. - $P(y_s|x_s) \neq P(y_t|x_y)$, when $X \rightarrow Y$ - $P(x_s|y_s) \neq P(x_t|y_t)$, when $Y \rightarrow X$