1. A cathode-ray tube and associated electronics connected to a computer's video output. A monitor may be either monochrome (black and white) or colour (RGB). Colour monitors may show either digital colour (each of the red, green and blue signals may be either on or off, giving eight possible colours: black, white, red, green, blue, cyan, magenta and yellow) or analog colour (red, green and blue signals are continuously variable allowing any combination to be displayed). Digital monitors are sometimes known as TTL because the voltages on the red, green and blue inputs are compatible with TTL logic chips. See also gamut, multisync, visual display unit. 2. A 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. 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. Unix man page: monitor(3). 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.