In the 1980s, with the various Unix vendors such as IBM, HP, Digital and SunMicrosystems each adding their own features, unix was in danger of becoming fragmented, making it harder to write portable programs. (Also the "BSD vs. System V" split.)
The name POSIX more correctly refers to a family of related standards: IEEE Std 1003.n and the parts of ISO/IEC 9945, although originally was just the original IEEE Std 1003.1-1988. POSIX means portable operating system interface. (the x is from unix ?)
POSIX is a large (some might even say unwieldy) standard (officially IEEE Standard 1003) specifying things such as the behaviour (arguments, etc) of standard programs and utilities that must exist on a system and C library functions. The theory is that if you write a program that conforms to POSIX standards, it will compile and run on any POSIX-compliant operating system.
/*
...
In early 2004, permission was granted for extracts from the (copyrighted) standard to be used in Linux manual pages.
Some parts of the standard (particularly the bits that are relevant to the Linux Kernel and UserSpace:
1003.1 defines the core functions and behaviours, as well as things like the standard ErrorMessages.
1003.1b includes real-time extensions.
1003.2 defines extra things such as standard RegularExpression syntax and behaviour, asynchronous I/O, and real time signal queues.
See also POSIXLY_CORRECT.
From "The Open Group Base Specification Issue 6":
Standardisation of the the pronounciation of the standards name I think is perhaps taking things too far.
POSIX® is a registered trademark of the Institute of Electrical and Electronic Engineers, Inc.
the X/Open `catgets' method, and the Uniforum `gettext' method. The `catgets' method indexes messages by integers; the `gettext' method indexes them by their English translations. The `catgets' method has been around longer and is supported by more vendors. The `gettext' method is supported by Sun, and it has been heard that the COSE multi-vendor initiative is supporting it. Neither method is a POSIX standard; the POSIX.1 committee had a lot of disagreement in this area.
Neither one is in the POSIX standard. There was much disagreement
in the POSIX.1 committee about using the `gettext' routines vs. `catgets' (XPG). In the end the committee couldn't agree on anything, so no messaging system was included as part of the standard. I believe the informative annex of the standard includes the XPG3 messaging interfaces, "...as an example of a messaging system that has been implemented..."
65 pages link to POSIX: