Wall Clock Time (also known as Real-World Time or Wall Time) is a term used to measure the running time of a program or process. It refers to the actual time spent from the start of program execution to the end, including all types of waiting and blocking time. This concept is relative to CPU time, which refers only to the time the program is actually executed on the processor, and does not include the time waiting for I/O operations or other resources. Wall Clock Time is a widely used term that helps developers and system administrators understand the overall performance of a program, including its actual use of system resources.