Substitution effect
In economics and particularly in consumer choice theory, the substitution effect is one component of the effect of a change in the price of a good upon the amount of that good demanded by a consumer, the other being the income effect. When a good's price decreases, if hypothetically the same "consumption bundle" were to be retained, income would be freed up which could be spent on a combination of more of each of the goods; thus, the new total consumption bundle chosen, compared to the old one, reflects both the effect on freed-up income (the income effect), and the effect of the change on the relative prices of the two goods (the substitution effect, one unit of one good now being traded for a different quantity of the other good, as the ratio of their prices has changed).