A function, macro or operating system service that can be called from multiple threads in a process at the same time. The function will always act in a well defined manner. The end results will be as if the function was called by each thread in turn, even though all of the threads were running the function at the same time. Some APIs have restrictions about how they can be called in order for them to be thread safe. See the API documentation for all APIs or system services that you use in a multi-threaded job.