POSIX Portable Operating System Interface
IEEE standard defining the Unix API — file system semantics, processes, shell, command-line tools. Allows portable code across Linux, BSD, macOS, AIX, Solaris.
First version 1988 (POSIX.1). IEEE Std 1003 is the umbrella. The Single UNIX Specification (SUS) is POSIX + extra requirements to be called "UNIX". macOS is POSIX-certified UNIX, Linux is POSIX-like but not officially certified. Windows POSIX subsystem existed in the NT era (abandoned), WSL gives POSIX now. fork(), pipe(), signal(), /dev/null, $HOME are all POSIX.