Section 2 (h) of the Indian Contract Act, 1872 says that an agreement enforceable by law is a contract.
Section 2 (e) provides the definition of agreement as every promise and every set of promises forming consideration for each other.
Section 2 (b) defines promise as when the person to whom the proposal is made signifies his assent thereto, the proposal is said to be accepted. A proposal, when accepted becomes a promise.
From these definitions, it is clear that:
a contract is an agreement;
an agreement is a promise
a promise is an accepted proposal.
OFFER + ACCEPTANCE= PROMISE
ENFORCEABILITY BY LAW= CONTRACT