A contract is a legally enforceable agreement between two or more parties that creates mutual obligations. It explicitly defines the rights and responsibilities of each party (e.g., the buyer and seller) involved in the transaction. The core purpose of a contract is to make the agreement's terms legally binding, providing a remedy if one party fails to perform its duties. This formal relationship, established through an agreement on rights and responsibilities, is the precise definition of a contract in the context of commerce and law.

B. Deal: This is an informal term for a transaction or agreement and lacks the implication of legal enforceability and formally defined rights inherent in a contract.

C. Concurrence: This refers to the act of agreeing or "a meeting of the minds" (mutual assent), which is an essential element of a contract, but not the relationship itself.

D. Union: This term describes the joining of entities, often in a political or labor context, and does not specifically define a commercial buyer-seller agreement.

1. National Contract Management Association. (2019). Contract Management Body of Knowledge (CMBOK) (6th ed.). § 1.2.1, The Definition of a Contract. The CMBOK defines a contract as "An agreement between two or more parties creating obligations that are enforceable or otherwise recognizable at law."

2. Cornell Law School Legal Information Institute (LII). (n.d.). Contract. In Wex Legal Dictionary/Encyclopedia. Retrieved from https://www.law.cornell.edu/wex/contract. The LII defines a contract as "an agreement between private parties creating mutual obligations enforceable by law," which establishes the respective rights and responsibilities.

3. Garrett, G. A. (2006). World-Class Contracting (4th ed.). CCH Incorporated. p. 11. The text states, "A contract is a promise or a set of promises for the breach of which the law gives a remedy... It is an agreement that creates legally enforceable duties and obligations." This aligns with the definition of a relationship with defined rights and responsibilities.