Definition of 'Complement'
A good or service that is used in conjunction with another good or service. Usually, the complementary good has little to no value when consumed alone but, when combined with another good or service, it adds to the overall value of the offering. Also, good tends to have more value when paired with a complement than it does by itself.
Investopedia explains 'Complement'
A product can be considered a complement when it shares a beneficial relationship with another product offering. In an economic sense, when the price of a good rises, the demand for its complement will fall because consumers don't want to use the complement alone.
For example, if the price of hot dogs rises so much that people stop consuming them, this will also cause a decrease in demand for hot dog buns. Because the price of hot dogs has an inverse relationship to the demand for hot dog buns, we call them complementary products.