What is the meaning of what is called “Contract A”?

In the leading case of Ontario v. Ron Engineering & Construction (Eastern) Ltd., 1981 CanLII 17 (SCC), [1981] 1 S.C.R. 111, at p. 9, (“Ron Engineering”), Estey J. explained the meaning of what is called “Contract A”: The tender submitted by the respondent brought contract A into life. This is sometimes described in law as a unilateral contract, that is to say a contract which results from an act made in response to an offer, as for example in the simplest terms, "I will pay you a dollar if you will cut my lawn". No obligation to cut the lawn exists in law and the obligation to pay the dollar comes into being upon the performance of the invited act. Here the call for tenders created no obligation in the respondent or in anyone else in or out of the construction world. When a member of the construction industry responds to the call for tenders, as the respondent has done here, that response takes the form of the submission of a tender, or a bid as it is sometimes called. The significance of the bid in law is that it at once becomes irrevocable if filed in conformity with the terms and conditions under which the call for tenders was made and if such terms so provide.