A resident monitor is a piece of system software that was used in many early computers from the 1950s to 1970s. It can be considered a primitive precursor to the operating system. The name is derived from a program which is always in the computers memory thus being "resident".
On a general-use computer using punched card input, the resident monitor governed the machine before and after each job control card was executed, loaded and interpreted each control card, and acted as a job sequencer for batch processing operations. The functions that the resident monitor could perform were: clearing memory from the last used program (with the exception of itself), loading programs, searching for program data and maintaining standard IO routines in memory.
Similar very primitive system software layers were typically in use in the early days of the later minicomputers and microcomputers before they gained the power to support full operating systems.