Contract

In law, a contract is a binding legal agreement that is enforceable in a court of law or by binding arbitration. That is to say, a contract is an exchange of promises with a specific remedy for breach.
Agreement is said to be reached when an offer capable of immediate acceptance is met with a "mirror image" acceptance (i.e., an unqualified acceptance). The parties must have the necessary capacity to contract and the contract must not be either trifling, indeterminate, impossible, or illegal. Contract law is based on the principle expressed in the Latin phrase pacta sunt servanda (usually literally "agreements are to be kept"). Breach of contract is recognized by the law and remedies can be provided.
As long as the good or service provided is legal, any oral agreement between two parties can constitute a binding legal contract. The practical limitation to this, however, is that only parties to a written agreement have material evidence (the written contract itself) to prove the actual terms uttered at the time the agreement was struck. In daily life, most contracts can be and are made orally, such as purchasing a book or a sandwich. Sometimes written contracts are required by either the parties, or by statutory law within various jurisdiction for certain types of agreement, for example when buying a house or land.
Contract law can be classified, as is habitual in civil law systems, as part of a general law of obligations (along with tort, unjust enrichment or restitution).
According to legal scholar Sir John William Salmond, a contract is "an agreement creating and defining the obligations between two or more parties".
As a means of economic ordering, contract relies on the notion of consensual exchange and has been extensively discussed in broader economic, sociological and anthropological terms (see "Contractual theory", below). In American English, the term extends beyond the legal meaning to encompass a broader category of agreements. (Wikipedia)