An adjective describing a program or library written such that should it be implemented in a thread-based system, it would not interfere with other threads (especially threads running instances of itself).

If a program/library was written in an unsafe manner, it might access global data structures without heed to other threads possibly accessing the same global data structures. If it were written in a safe manner, it would somehow (through mutexes, semaphores or the like) protect areas of code that accessed common data structures between possibly co-existing threads.