在英语中，合同一般称为Contract，协议一般称为Agreement.

何谓“contract”？
1999年中国《合同法》第二条对contract定义为： A contact in this Law refers to an agreement establishing， modifying and terminating the civil rights and obligations between subjects ofequal footing， that is， between natural persons， legal persons or other organizations“。根据这一定义，合同平等主体之间设立的确定民事权利和义务的协议。
Steven H. Gifts编著的“Law Dictionary”中将contract 定义为“contract is a promise， or a set ofpromises， for breach of which the law gives remedy， or the performance of the which thelaw in some way recognize as a duty.”根据这一定义，合同是一种承诺，违反承诺可以得到法律救助，某种意义上法律将履行该承诺看做是一种补偿。
L.B Curzon 在其编撰的字典“A Dictionary of Law”给contract的定义：“Contract is a legally bindingagreement”根据这一定义，合同就是有法律约束力的协议。
综合起来，有一个相同点，就是“Contract is an agreement”，即可将合同说成是“An agreement whichbinds the parties concerned”或者说合同说成是“An agreement which is enforceable by law”， 也可以说：Contracts are promises that the law will enforce.

何谓“Agreement”？
L.B “A consensus of mind， or evidence of such consensus， in spoke or written form， relating to anything done or to be done.”
根据这一定义，协议是对已经做或准备做的相关事宜，经过谈判、协商后取得一致意见，以口头或书面形式做出的约定。
Black "Law Dictionary"有两个定义。
一个是：“A concord of understanding and intention betweentwo or more parties with respect to the effect upon their relative rights and duties， of certainpast or future facts or performance”根据这一定义，协议即双方或多方京某些过去或将来某些事实的相关权利、义务或相关权利、义务的履行而达成的一致理解和愿望。
另一个是：“The consent of two or more persons concurring respecting the transmission of someproperty， right or benefits， with the view of contacting an obligation， a mutualobligation.”根据这一定义，协议即两个或多个当事人，为了约定单方责任或相互责任，就财产权利、利益的转移取得的一致同意。