POSIX

POSIX is a technical acronym and a proper noun referring to a specific set of standards. It is used almost exclusively in the context of software engineering, systems programming, and operating system design to denote a level of interoperability.

Because it refers to a singular, defined standard, it is treated as an uncountable noun. It does not have a plural form and is typically used as an attributive noun (e.g., POSIX threads, POSIX compliance) to describe software that adheres to these specifications.