Cross-platform software is software that is available for more than one computer platform. Each version of a cross-platform application may have slight differences in the user interface and feature set, but they are fundamentally the same program capable of opening the same files and performing the same functions. The opposite of a cross-platform app is a "native" app, developed exclusively for a single platform.
Some cross-platform apps are available for both desktop and mobile platforms. In these cases, the mobile version of the app may have a limited feature set, but is compatible with the same documents and capable of most of its functions. For example, Microsoft Word is a cross-platform app with a full-featured desktop version available for Windows and macOS; it also offers a mobile version for iOS and Android and a web app that can run in a browser.
NOTE: Hardware peripherals compatible with multiple platforms, like USB mice and keyboards, may also be considered cross-platform.