Multithreading / Multiprocessing

Multithreading refers to the general task of running more that one thread of execution within an operating system. Multithreading is more generically called multiprocessing, which can include multiple system processes (a simple example on Windows would be, e.g., running Internet Explorer and Microsoft Word at the same time), or it can consist of one process that has multiple threads within it.

Multithreading (or should I say, multiprocessing) is a software concept. Practically any Turing-complete CPU can perform multithreading, even if the computer only has one CPU core and that core does not support hyperthreading. In order to support multiprocessing, the CPU will interleave execution of different threads of execution, by executing one, then another, then another, where the operating system will divide up the time available into "slices" and give a roughly equal amount of time to each thread (the time doesn't have to be equal, but that's typically how it's done unless a process requests a higher priority).

Note that, whenever there are more software threads of execution trying to execute at any given time than there are available hardware (simultaneous) threads of execution, then these software threads will be "interleaved" among the available cores. In the case of a "uniprocessor" (one CPU core with no hyperthreading), if you have more than one software thread, they will always be interleaved. If you have a 4-core CPU with hyperthreading, that's 8 "hardware threads", meaning the CPU can execute 8 simultaneous threads of execution at the same instant, so if you had 8 software threads trying to run, they could all run at once; but if you had 9 software threads, one of the hardware threads would have to interleave a pair of threads (the exact pair of threads chosen would depend on the operating system's scheduler implementation).