Final answer:
Complementary goods or services are those that are typically used together, with the consumption of one enhancing that of the other. Price changes in one can directly affect the demand for the other.
Step-by-step explanation:
Goods or services that are highly interdependent, or if one significantly modifies another, are typically referred to as complements. These are goods or services that are often used together, meaning that the consumption of one tends to enhance the consumption of the other. This is explained by the concept of ceteris paribus, which means 'other things being equal.' For instance, when the price of one complement like golf club increases, following the law of demand, the demand for its complement, golf balls, would decrease. Similarly, a decrease in the price of skis would likely increase the demand for complementary goods or services like ski resort trips. Thus, the demand for complementary goods is interlinked and subject to change in response to price movements or other impacting factors.