Voluntary but conditional promise submitted by a buyer or seller (offeror) to another (offeree) for acceptance, and which becomes legally enforceable if accepted by the offeree.
A explicit proposal to contract which, if accepted, completes the contract and binds both the person that made the offer and the person accepting the offer to the terms of the contract. http://www.duhaime.org/LegalDictionary/O/Offer.aspx Terms and conditions under which an offer is made, such as quantity, price, discounts, delivery date, shipping costs http://www.businessdictionary.com/definition/offer.html Acceptance of an offer is the final and unequivocal expression of assent to another's offer to contract. http://www.duhaime.org/LegalDictionary/A/Acceptance.aspx "Acceptance means the signification by the offeree of his willingness to enter into a contract with the offeror on the terms offered to him by the latter. Without an acceptance there can be no contract ...."

"In the ordinary case ... there is a manifestation of willingness to be bound by one party followed by a manifestation of an assent by the other by words or conduct, generally and usefully called offer and acceptance."

"Each of (these) definitions contemplates a willingness, either in words or by conduct, to be bound by the terms of the offer. But each definition also entails a manifestation, or a signification, or an indication of such willingness." Book 9(1) of Halsbury's Laws of England, 4th Edition (2007)
“termination of offer”, and can happen in a number of ways, for a variety of reasons. Termination of offer is different from “termination of contract”, as a contract has not yet been fully formed in a termination of offer.

How is an Offer Termination Accomplished?

There are many different ways that an offer for a contract can be terminated. These can involve either actions by the offeror, offeree, or other intervening circumstances. Termination of offer may occur by:

Rejection: the offeror usually needs to clearly communicate their intention to reject the offer to the offeror. Rejection or revocation of an offer generally becomes effective when the offeree learns of it

How is an offer terminated?

There are a number of ways for an offer to be terminated. They are events that may occur after an offer has been made which bring it to an end so that it can no longer be accepted. An offer is terminated in the following circumstances:

Revocation

Rejection

Lapse of time

Conditional Offer

Operation of law

Death

Acceptance

Illegality

REVOCATION

Revocation means an offer is withdrawn by the offerer. The general rule was established in Payne v Cave [1] that an offer can be revoked at any time before acceptance takes place.
However, the revocation must be communicated effectively directly or indirectly to the offeree before acceptance [2] . This is supported by Byrne v Van Tienhoven [3] , where the withdrawal of an offer sent by telegram was held to be communicated only when the telegram was received. [4] Further, sufficient communication does not need to be made by the offeree personally but through a third party in Dickinson v Dodds [5] . In Routledge v Grant [6] , the offer may still be able to withdraw even if it specifically stated that it would remain open for a fixed period when such promise to leave an offer open was not supported by any consideration given by the offeree. [7] However, once the offeree accepted the offer by post, namely, letter, the postal rule would strictly apply and would not permit such withdrawal. Contrary, once the offer has been accepted and acted upon, it cannot be revoked, the incompliance of it would be a breach of contract.
REJECTION
An offer is terminated when the offeree communicates his rejection to the offeror. Hence, the offeree making a counter-offer and introduces a new offer amounts to a rejection of the original offer. In Hyde v Wrench [9] , Lord Langdale held that the counter offer offered by the offeree terminated the original offer. Hence, “…thereby rejected the offer previously made by the defendant (the offerer).” However, this should be distinguished from the situation when the offeree merely seeks further information from the offeror, and does not make a counter-offer. In Stevenson, Jacques v McLean [10] a mere inquiry would not be considered as rejection.
Lapse of Time: the offeree generally has a “reasonable amount of time” in which to respond to the offer after they learn of it. This amount of time may vary depending on the subject matter of the contract, as well as the prior dealings between the parties.
As it would be impracticable if an offer could be accepted after an unreasonable delay on the part of the offeree [11] , the court stated in Ramsgate Victoria Hotel v Montefiore [12] that an offer will lapse if it is open for a specific length of time and that time limit expires. Where there is no express time limit, an offer is normally open only for a reasonable time. The length for a reasonable time will depend on the circumstances of the case with respect to offers involving other types of subject matter, definition of a reasonable time depends upon the demand for the subject matters and upon the volatility of its price.

Death/Incapacity/Disability of the other Party: If the offeror becomes deceased, incapacitated, or disabled before the contract offer is accepted, it will usually terminate the offer.
The death of either the offeror or the offeree will cause such termination: the right to accept an ordinary offer is not transferable. The unaccepted offer of a deceased person cannot be converted into a contract binding upon his estate. [18] In Dickinson v Dodds [19] Mellish LJ stated ‘if a man who makes an offer dies, the offer cannot be accepted after he is dead.’ On the other hand, in Reynolds v Atherton [20] Warrington LJ stated that an offer “made to a living person who ceases to be a living person before the offer is accepted…is no longer an offer at all”

Conditional Acceptance: If the offeree makes an acceptance that is conditional (i.e., “I’ll only accept this offer if I can also have a free product”), this is not viewed as an acceptance of the offer, but rather a counter-offer.

Illegality: If performance of contract has become illegal after the offer has been made, the offer will terminate, and the offeree can no longer legally accept the offer. a change in the law which makes a potential contract illegal will terminate an offer, since courts will not enforce an illegal contract. [22]

There may be other reasons that a contract offer may be terminated, depending on the type of contract and the different laws involved. For example, if the subject matter of the contract is irretrievably destroyed, the destruction may serve to terminate any offers that were in existence at the time. http://www.legalmatch.com/law-library/article/offer-termination-in-contracts.html In conclusion, offer can be terminated by Revocation, Rejection, Lapse of time, Conditional Offer, Operation of law, Death, Acceptance and Illegality