Synallagma
Synallagma, from the Ancient Greek for “mutual agreement,” is a concept in the law of obligations denoting the mutual interdependence of the parties’ obligations in a bilateral contract, where one party’s performance is directly tied to the counter-performance of the other party—the principle of do ut des, “I give so that you may give.”
In civil-law systems, a synallagmatic contract is one in which each party is obligated to provide something to the other party. Examples of synallagmatic contracts include sales contracts, service contracts, and lease agreements.