POSIX Threads is a standard API (application programming interface) for creating and managing threads in a multi-threaded programming environment. It is particularly used in Unix-like operating systems such as Linux and macOS. The implementation of POSIX Threads provides a uniform interface for creating, synchronizing, and communicating between threads in a program. A person with a Specialized Skill in POSIX Threads would have expertise in using the API to optimize program performance and efficiency in a multi-threaded environment.