Able to run on more than one platform. Differnet people have different idea's about what CrossPlatform means. Most Unix people consider CrossPlatform to mean that it will run on other Unix platforms and probably implements/uses POSIX interfaces. MicrosoftCorporation often say "CrossPlatform" when they mean it runs on Windows 95, Windows 2000, and maybe Windows CE.
5 pages link to CrossPlatform: