The Free On-line Dictionary of Computing (30 December 2018):
Sharing a single CPU between multiple tasks (or
"threads") in a way designed to minimise the time required to
switch threads. This is accomplished by sharing as much as
possible of the program execution environment between the
different threads so that very little state needs to be saved
and restored when changing thread.
Multithreading differs from multitasking in that threads
share more of their environment with each other than do tasks
under multitasking. Threads may be distinguished only by the
value of their program counters and stack pointers while
sharing a single address space and set of global
variables. There is thus very little protection of one
thread from another, in contrast to multitasking.
Multithreading can thus be used for very fine-grain
multitasking, at the level of a few instructions, and so can hide
latency by keeping the processor busy after one thread issues a
long-latency instruction on which subsequent instructions in that
A light-weight process is somewhere between a thread and a
TL0 is an example of a threaded machine language.
Dataflow computation (E.g. Id and SISAL) is an extreme
form of multithreading.