Semaphore
A semaphore is a synchronization counter with wait/P (decrement, block if zero) and signal/V (increment, wake a waiter). It limits how many threads may hold a resource at once.
A binary semaphore (count 0/1) overlaps a mutex, though mutexes usually add ownership and recursive-lock rules. Counting semaphores gate pools: three identical buffers, three permits.
In a Petri net, the semaphore is a place whose tokens are permits. Misordered waits across semaphores are a standard deadlock recipe; forgetting a signal starves everyone behind you.