The ratio between the proportional change in demand for a good and the proportional change in the price of a different good. This is calculated assuming that the price of the good itself is constant. Thus if qx is the quantity of good x, and py is the price of good y, the cross-price elasticity of demand is given by
If εxy > 0, x and y are substitutes; if εxy < 0, x and y are complements.