Uniqueness of subject matter refers to the distinct nature of certain goods or services that makes them irreplaceable or one-of-a-kind, which is essential in the context of specific performance. This principle implies that when a party enters into a contract involving unique items, such as rare art or real estate, the law recognizes that monetary damages may not suffice to remedy a breach, thus enabling the injured party to seek specific performance to compel the other party to fulfill their contractual obligations.