Conforming to the basic architecture and principles of the Unix operating system. Unix-like operating systems, such as Linux, may be POSIX compliant, which means that applications communicate with the OS via a standard programming interface (API).

The phrase "Unix-like operating system" is meant to exclude Unix operating systems such as Sun's Solaris and HP's HP-UX because they, as well as others, are compliant with one of the UNIX specifications governed by The Open Group. Any Unix operating system that does not comply is a Unix-like operating system. See Unix, Open Group and Single UNIX Specification.