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.

Almost no CrossPlatform applications are not truely CrossPlatform, most perform better, integrate better or are easier to use on a particular platform, mainly because each platform makes a different set of assumptions, and making the union of assumptions in an application requires N^2 implementation effort and making the intersection leaves you with a painfully low level of functionality.

wxWindows is a GUI toolkit that allows your source code to be very portable - often you only need to re-compile the source code against a different port of the toolkit to get native look-and-feel on GTK, Motif, MacOS 9, MacOSX, and WIN32 (and some other graphical platforms). I think you can even compile and link against the windows port of GTK!

Many Java programs are relatively CrossPlatform, including the Eclipse IDE.