If the substitution effect and the income effect of a price change move in opposite directions for a particular good,then the good must be an inferior good.

Substitution Effect

The change in the quantity demanded of a good that results from a change in price, making the good more or less expensive relative to other goods.

Income Effect

The alteration in the consumption habits of a person or an economy due to a variation in actual income.

Inferior Good

A type of good for which demand decreases when income increases, and vice versa, unlike normal goods where demand increases with an increase in income.