The ability of a program to run on more than one computer operating system, such as Mac, Windows, or Linux. It's also the ability of a file or Web page to be accurately viewed on a variety of computer operating systems. Before the Internet became so commercially popular, many programs or files were not compatible with other operating systems (and you couldn't share work with anyone unless they were on the same system). But because the Internet is such an open standard for communication, programs had to become cross-platform so we could all view each other's information. This term is also used to describe people who know both Mac and Windows (and/or Linux or other operating systems). You'll hear it used like this: "I'm cross-platform. I know both Mac and PC."