This is a word that is (unfortunately) used fairly frequently in the computer and audio worlds. In the broader sense, proprietary means that a concept or product is unique to, and the property of a manufacturer or company. More commonly, proprietary refers to a manufacturer designing a product to only work with other products from that same manufacturer. For example, a manufacturer might make a synthesizer that can only save patch information to specific, specially designed RAM cards, rather than to more universal PCMCIA cards, floppy disks, or whatever. In order to save that synth’s information you would be required to use the proprietary cards available only from that manufacturer. While the word “proprietary” is often given a negative connotation, keep in mind that building gear around proprietary designs and options allows a manufacturer to implement features that might not be possible if everything were standardized and generic.