1. The type of computer on which a given operating system or application runs. Synonym hardware platform. 2. The operating system in use on a given computer. Synonym operating system platform. 3. The application program in use on a given computer and operating system. Synonym application platform. Note 1: The term platform, usually with some kind of accompanying qualifying verbiage, may also be applied to any combination of the foregoing. Note 2: The term cross-platform may be used to characterize an application program or operating system that may be run on more than one platform.