Definition of Firm offer
As defined by U.C.C. is an offer by a merchant to buy or sell goods in a signed writing which by its terms give assurance that it will be held open. Such is not revocable for lack of consideration during the time stated or if no time is stated for a reasonable time, but in no event may such period of irrevocability exceed three months; but any such term of assurance on a form supplied by the offeree must be separately signed by the offeror. U.C.C. § 2-205. A binding, definite offer that is irrevocable for an agreed upon time.
That's the definition of Firm offer in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.