an interest in personal property or fixtures which secures payment or performance of an obligation. "Security interest" includes any interest of a consignor and a buyer of accounts, chattel paper, a payment intangible, or a promissory note in a transaction. "Security interest" does not include the special property interest of a buyer of goods on identification of those goods to a contract for sale, the right of a seller or lessor of goods to retain or acquire possession of the goods is not a "security interest", but a seller or lessor may also acquire a "security interest". The retention or reservation of title by a seller of goods notwithstanding shipment or delivery to the buyer is limited in effect to a reservation of a security interest