Final answer:
A complementary good is used with another good, enhancing the consumption of each other. They have a negative cross-price elasticity of demand, meaning a price increase in one leads to a decrease in demand for the other.
Step-by-step explanation:
A complementary good is one that is used together with another good. These goods are known as complements because they are often used together so that consumption of one good tends to enhance the consumption of the other. In economic terms, under the assumption of ceteris paribus, or other things being equal, complementary goods have a negative cross-price elasticity of demand. That means if the price of one good increases, the demand for its complementary good will decrease, and vice versa.
For example, if good A is a complement for good B, such as coffee and sugar, then a higher price for B will likely mean a decrease in the consumption of A. This is because these two goods are often used together, and a change in the cost of one affects the demand for the other.