This is best described as a standard that a computer system can be developed Operating systems such as Windows and DOS are examples of platforms that work on the PC platform only Likewise, MacOS can only be run on an apple Mac computer When you have a "Cross Platform" you are referring to hardware or software that can be used on BOTH PC's and Macintosh computers
If an application is cross platform it means it will run on computers irrespective of the operating system Usually cross platform usually refers to the two main operating systems for PCs: Windows and Apple Mac