A monitor may be
1. A computer display
2. A concurrent programming language construct which encapsulates variables, access procedures and initialisation code within an abstract data type. The monitor's variable may only be accessed via its access procedures and only one process may be actively accessing the monitor at any one time. The access procedures are critical sections. A monitor may have a queue of processes which are waiting to access it. Monitors were invented by C. A. R. Hoare.
3. A hardware device that measures electrical events such as pulses or voltage levels in a digital computer.
4. To oversee a program during execution. For example, the monitor function in the Unix C library enables profiling of a certain range of code addresses. A histogram is produced showing how often the program counter was found to be at each position and how often each profiled function was called.
5. A control program within the operating system that manages the allocation of system resources to active programs.
6. A program that measures software performance.
7. USS Monitor, the ironclad warship.
The first six portions of this article are based on monitor at FOLDOC, used with permission.