Define an "offer" in contract law.

A valid contract must consist of agreement (offer and acceptance), as well as intention to create legal relations and consideration. Treitel defines an offer as an “expression of willingness to contract on specified terms made with the intention that it is to become legally binding as soon as it is accepted by the person to whom it is addressed”. An offer must be: (1) clear and certain, (2) communicated and (3) must show intention to enter into a legal relationship (objectively ascertained). // Regarding the first of the above criteria: in Gibson v Manchester CC, following a change of policy which permitted the purchase of council houses by their existing tenants, Mr Gibson filled out a form to enquire as to the cost of his own. The policy was scrapped; Mr Gibson claimed that his actions had created a contract. The House of Lords held that no offer had been made; the terms “may be prepared to sell” were too vague and unclear. Offers must show intention to be bound (John D.R. Leonard v Pepsico Inc - a U.S. case where it was held that Pepsi’s humorous advertisement had not constituted a unilateral offer as it was clearly in jest). // An offer must also be distinguished from an invitation to treat, which represents the first step in a series of negotiations, not an offer. These can include adverts (Partridge v Crittenden, a newspaper advert concerning the sale of wild birds) and auctions (Payne v Cave). There are two exceptions here: firstly, clear and certain adverts by manufacturers with limitless supplies (Grainger v Gough, where a wine advert from the supplier was held in obiter to be an offer). The second exception concerns unilateral offers where one party makes an offer in terms calling for an act to be performed by one or more other parties. An example is in Carlill v Carbolic Smoke Ball, where the company producing the smoke ball stated in an advert that those using the product as stated that still caught flu would be eligible to receive £100. It was held that there had been a clear, prescribed act, and that Mrs Carlill’s performance of this constituted acceptance. Further invitations to treat also include the display of goods with price marked (Fisher v Bell, concerning a flick-knife) and invitations to tender (Spencer v Harding), except where there is a clear undertaking to accept the highest or lowest bid/offer (Blackpool & Fylde Aero Club v Blackpool BC). // An offer must be communicated - written, spoken or via conduct (Taylor v Laird, where the conduct was held to be too vague and the offeree had not been given the opportunity to accept).