Multi-tasking

Multitasking operating systems are a subset of operating systems - now, by far the predominant form - which allow multiple programs to run at the same time.

There are two principal types: preemptive multitasking, in which computations can be (unkownst to them) interrupted in the middle of their work to allow other computations to run, and cooperative multitasking, in which a running computation has to explicitly 'give up' the machine, in order for another computation to run.

Note that this relinquishment may be a side effect of something the computation does; e.g. it may call for some input or output, and while waiting for that to happen, some other computation may be run.