POSIX
In plain terms
A standard defining a portable interface for Unix-like systems.
Going deeper
POSIX specifies APIs and shell behavior so software written to it runs across compliant operating systems. It enables portability.
Under the hood
Covers system calls, threads, and utilities; compliance eases porting between Unix-like OSes.
Examples
Cross-platform code, Unix/Linux/macOS compatibility, shell standards, threads.
Think of it like...
Like a common language standard so programs speak the same dialect across systems.