1) In computers, a platform is an underlying computer system on which application programs can run. On personal computers, Windows 2000 and the Mac OS X are examples of two different platforms. On enterprise servers or mainframes, IBM's S/390 is an example of a platform.
A platform consists of an operating system, the computer system's coordinating program, which in turn is built on the instruction set for a processor or microprocessor, the hardware that performs logic operations and manages data movement in the computer. The operating system must be designed to work with the particular processor's set of instructions. As an example, Microsoft's Windows 2000 is built to work with a series of microprocessors from the Intel Corporation that share the same or similar sets of instructions. There are usually other implied parts in any computer platform such as a motherboard and a data bus, but these parts have increasingly become modularized and standardized.
Historically, most application programs have had to be written to run on a particular platform. Each platform provided a different application program interface for different system services. Thus, a PC program would have to be written to run on the Windows 2000 platform and then again to run on the Mac OS X platform. Although these platform differences continue to exist and there will probably always be proprietary differences between them, new open or standards-conforming interfaces now allow many programs to run on different platforms or to interoperate with different platforms through mediating or "broker" programs.
2) A platform is any base of technologies on which other technologies or processes are built.