1. The ability of an operating system to run more than one program, or task, at a time. A cooperative multitasking OS, like Windows 95/98, requires one application to voluntarily free up resources upon request so another application can use it. A preemptive multitasking OS, such as UNIX/Linux, Windows NT/2000 or OS/2, frees up resources when ordered to by the operating system, on a time-slice basis, or a priority basis, so that one application is unable to hog resources when they are needed by another program. (Also, see Multithreading and Time-sharing.) From I-gloss

2. the execution of two or more processes at the same time. From Linux Guide @FirstLinux