The definition of multitasking

Some OSes have built into them the ability to do several things at once. This is called multitasking, and has been in use since the late sixties / early seventies. Since this ability is built into the software, the overall system will be slower running two things at once than it will be running just one thing. A system may have more than one processor built into it though, and such a system will be capable of running multiple things at once with less of a performance hit.