Belgian Civil Code
The contract is an agreement by which one or more people undertake, towards one or more others, to give, to do or not to do something.
Civil Code of Quebec
Article 1378. The contract is an agreement of will, by which one or more persons bind themselves to one or more others to perform a service.
Swiss Code of Obligations Obligations resulting from a contract
Restatement (Second) of Contract
The Restatement (Second) of Contract gives in its §1 this definition of the contract
A contract is a promise or a set of promises for the breach of which the law gives a remedy, or the performance of which the law in some way recognizes as a duty.