What is the difference between an offer and a proposal?

According to the Federal Acquisition Regulation (FAR), “offer” means a response to a solicitation that, if accepted, would bind the offeror to perform the resultant contract. Responses to requests for proposals (negotiation) are offers called “proposals.” For the purpose of the U.S. DOT SBIR program, Phase I submissions are referred to as “offers,” while Phase II and IIB submissions are referred to as “proposals.”