A gentlemen's agreement litterally agreement among gentlemen is an informal agreement between two or more parties.
It can be written, oral, or implicit by communicating a non-verbal agreement, through an agreement or mutual benefit.
The essence of a gentlemen's agreement lies in the fact that it rests on the honor of the parties in relation to its realization, rather than in the fact that they would be liable of anything.
It is, therefore, different from a contract or from an agreement, which can be enforced if necessary.
The first known use of this phrase dates back to 1888, in the Report of the Accounting Officers of the Railway, published by the Association of Accountants of the American Railways (p. 337).