From LQWiki

1) The term platform can be used to describe the hardware of a computer system, particularly with reference to the operating systems it is able to run. For example; many Linux distributions are intended to run on the x86 platform, but there are also versions available for PPC, Sparc and Alpha.

2) An application or operating system that runs on top of hardware, and on top of which other software may run. Java can be defined as a platform. Software that runs on several operating systems (Mozilla, for example) can be described as "cross-platform".