Mutex
Also known as: Mutual exclusion, Lock
A mutex (mutual exclusion) is a lock that lets at most one holder use a shared resource at a time. Acquire before the critical section, release after; everyone else waits.
In code this is an OS or library lock (pthread_mutex, std::mutex, Java synchronized). Holding too long hurts throughput. Forgetting to release, or locking in inconsistent order across threads, is a classic path to deadlock.
In a Petri net, the same idea is a single resource token in a place: the enter transition consumes it, the exit transition returns it. A semaphore generalizes the count; a mutex is the count-1 case with ownership conventions.