CPU time (or processing time) is the amount of time for which a central processing unit (CPU) was used for processing instructions of a computer program or operating system, as opposed to elapsed time, which includes for example, waiting for input/output (I/O) operations or entering low-power (idle) mode. The CPU time is measured in clock ticks or seconds. Often, it is useful to measure CPU time as a percentage of the CPU's capacity, which is called the CPU usage.
Programs and applications normally do not use the processor 100% of the time that they are running; some of that time is spent on I/O operations and fetching and storing data on the RAM or storage device. The CPU time is only when the program actually uses the CPU to perform tasks such as doing arithmetic and logic operations.