Final answer:
A semaphore is a synchronization data type used in concurrent programming to control resource access by blocking processes or threads until a condition is met.
Step-by-step explanation:
A semaphore is a data type that is used to block a process or thread until a particular condition is true. Semaphores are used in concurrent programming to control access to resources and to synchronize the execution of threads. They act as a signal mechanism, where a thread will wait (block) on a semaphore if the resource it is trying to access is already being used by another thread and will only proceed when it is allowed to do so, ensuring that the resource is being used safely and without conflicts.
Semaphores can be counting semaphores, which allow a certain number of threads to access a resource concurrently, or binary semaphores (also known as mutexes), which restrict access to only one thread at a time. When a thread finishes using the resource, it signals the semaphore, potentially unblocking a waiting thread. This behavior is pivotal in achieving the correct operation of many concurrent systems, from operating systems to complex applications.