gentlemen's agreement

from the GNU version of the Collaborative International Dictionary of English.

• An agreement binding only as a matter of honor; often, specif., such an agreement among the heads of industrial or merchantile enterprises, the terms of which could not be included and enforced in a legal contract.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a personal agreement based on honor and not legally binding