Lock / semaphore
Can involve any number of tasks
Typically used to serialize (protect) access to global data or a section of code. Only one task at a time may use (own) the lock / semaphore / flag.
The first task to acquire the lock "sets" it. This task can then safely (serially) access the protected data or code.
Other tasks can attempt to acquire the lock but must wait until the task that owns the lock releases it.
Can be blocking or non-blocking