[adrian_b]

You use "parallelism" with a different meaning than me.

I use "parallel" with its original meaning "one besides the other", i.e. for spatial parallelism.

You use "parallel" with the meaning "simultaneous in time", because only with this meaning you can call pipelining as a form of parallelism.

You are not the only one who uses "parallel" for "simultaneous in time", but in my opinion this is a usage that must be discouraged, because it is not useful.

If you use "parallel" with your meaning, you must find new words to distinguish parallelism in space from parallelism in time. There are no such words in widespread use, so the best that you can do is to say "parallel in space" and "parallel in time", which is too cumbersome.

It is much more convenient to use "parallel" only with its original meaning, for parallelism in space, which in the case of parallel execution requires multiple equivalent execution units (unlike pipelined execution, which in most cases uses multiple non-equivalent execution units).

When "parallel" is restricted to parallelism in space, pipelining is not a form of parallelism. Both for pipelining and for parallelism there are multiple execution units that work simultaneously in time, but the stream of data passes in parallel through the parallel execution units and in series through the pipelined execution units.

With this meaning of "parallel", one can speak about "parallel execution" and "pipelined execution" without any ambiguity. It is extremely frequent to have the need to discuss about both "parallel execution" and "pipelined execution" in the same context or even in the same sentence, because these 2 techniques are normally combined in various ways.

When "parallel" is used for simultaneity in time it becomes hard to distinguish parallel in space execution from pipelined execution.

[mafribe]

The pipeline stages (say: fetch, decode, execute, memory access, register write back), are organised "parallel in space" as transistors on chip. The point of having a pipeline is so the stages can execute "parallel in time".

More generally, parallel in space is interesting because it is a necessary precondition for parallel in time.

[adrian_b]

In its original meaning, which is still the meaning used in mathematics and physics, "parallel" provides more information than just saying that the parallel things are located in different places in space. Such an information can be provided by other words.

One should not say that the pipeline stages are parallel, when the intended meaning is that they are separate or distinct, which is the correct precondition for their ability to work simultaneous in time.

"Parallel" says about two things that they are located side-by-side, with their front-to-back axes aligned, which is true for parallel execution units where the executions of multiple operations are initiated simultaneously in all subunits, but it is false for pipelined execution units, where the executions of multiple operations are initiated sequentially, both in the first stage and in all following stages, but the initiation of an execution is done before the completion of the previous execution, leading to executions that are overlapped in time.

The difference between parallel execution and pipelined execution is the same as between parallel connections and series connections in any kind of networks, e.g. electrical circuits or networks describing fluid flow.

Therefore it is better if the terms used in computing remain consistent with the terms used in mathematics, physics and engineering, which have already been used for centuries before the creation of the computing terminology.