English law has no formal definition of the contract. In the absence of a Code it has not needed one, moreover any such definition is not a part of the law itself. Very generally it can be said that by the contract is usually understood a commercial agreement which gives rise to obligations enforced and recognised by law.
In Brogden v Metropolitan Railway (1877) 2 App Cas 666, per Lord Blackburn at p.692:
…where it is expressly or impliedly stated in the offer that you may accept the offer by posting a letter, the moment you post the letter the offer is accepted. You are bound from the moment you post the letter, not, as it is put here, from the moment you make up your mind on the subject.
Per Lord Wilberforce in Brinkibon Ltd v Stahag Stahl und Stahlwarenhandelsgesellschaft mbH  1 All ER 293 at p.295:
[A] contract is formed when acceptance of an offer is communicated by the offeree to the offeror. And if it is necessary to determine where a contract is formed (as to which I have already commented) it appears logical that this should be at the place where acceptance is communicated to the offeror. In the common case of contracts, whether oral or in writing inter praesentes, there is no difficulty; and again logic demands that even where there is not mutual presence at the same place and at the same time, if communication is instantaneous, for example by telephone or radio communication, the same result should follow.