1. The ability of an operating system to concurrently run programs that have been divided into subcomponents, or threads. Multithreading, when done correctly, offers better utilization of processors and other system resources. Multithreaded programming requires a multitasking/multithreading operating system, such as UNIX/Linux, Windows NT/2000 or OS/2, capable of running many programs concurrently. A word processor can make good use of multithreading, because it can spell check in the foreground while saving to disk and sending output to the system print spooler in the background. (Also, see Thread.) From I-gloss

2. native kernel support for multiple independent threads of control within a single process memory space. From Linux Guide @FirstLinux