As far as I know, platform agnostic is a valid term. I could be wrong since I'm not a native English speaker. But yes, I meant cross-platform way.
http://en.wikipedia.org/wiki/Platform_agnostic

Another approach that is used is to depend on pre-existing software that hides the differences between the platforms—called abstraction of the platform—such that the program itself is unaware of the platform it is running on. It could be said that such programs are platform agnostic. Programs that run on the Java Virtual Machine (JVM) are built in this fashion.