The history of threads

The notion of a thread, as a
sequential flow of control, dates back to 1965, at least, with the
Berkeley Timesharing System.  Only they weren't called threads at that
time, but processes [Dijkstra, 65].  Processes interacted through
shared variables, semaphores, and similar means.  Max Smith did a
prototype threads implementation on Multics around 1970; it used
multiple stacks in a single heavyweight process to support background
compilations.

Perhaps the most important progenitor of threads is the programming
language PL/I, from about the 1965 time frame.  The language as
defined by IBM provided a `CALL XXX (A, B) TASK;' construct, which
forked a thread for XXX.  It is not clear whether any IBM compiler
ever implemented this feature, but it was examined closely while
Multics was being designed; it was decided that the TASK call as
defined didn't map onto processes, since there was no protection
between the threads of control.  So Multics took a different
direction, and the TASK feature was removed from PL/I by IBM in any
case, along with the ABNORMAL attribute and lots of other weird stuff.

Then came Unix, in the early 1970s.  The Unix notion of a `process'
became a sequential thread of control *plus* a virtual address space
(incidentally, the Unix notion of a process derived directly from the
Multics process design [Saltzer, 66]).  So `processes', in the Unix
sense, are quite heavyweight machines.  Since they cannot share memory
(each has its own address space), they interact through pipes,
signals, etc).  Shared memory (also a rather ponderous mechanism) was
added much later.

After some time, Unix users started to miss the old processes that
could share memory.  This led to the `invention' of threads: old-style
processes that shared the address space of a single Unix process.
They also were called `lightweight', by way of contrast with
`heavyweight' Unix processes.  This distinction dates back to the very
late 70s or early 80s, i.e. to the first `microkernels' (Thoth
(precursor of the V-kernel and QNX), Amoeba, Chorus, the
RIG-Accent-Mach family, etc).

On a side note, threads have been in continuous use in
telecommunications applications for quite a long time.