If I want to describe two things happening at the same time, I can use the words parallel or concurrently. In some cases, simultaneously would also work. More suggestions are available in the opposite question, already answered.

In technical writing, most of the time, it is sufficient to identify a process or thread that is parallel; a lack of parallelism is assumed in absence of specificity. But in some cases, the ordinary expectation for something is parallel, so the reverse requires denotation.

The word sequentially implies some of the same things as concurrently. But I'd really like a word that denotes the opposite and can be used in the same way as concurrently.

Here's a few examples:

  • "That procedure runs concurrently, so the system doesn't wait for it to finish before moving on."
  • "That procedure runs ???, so the system waits for it to finish before moving on."

  • "The math library does all of its processing concurrently, so matrix multiplication is faster on a multi-core machine."

  • "The math library does all of its processing ???, so it doesn't matter how many cores you have."

  • "The backup runs concurrently with the import, so the system doesn't experience any downtime."

  • "The backup runs ??? with the import, so the system will be down until it completes."

The audience can assumed to be reasonably technical, though not necessarily domain experts in multithreaded programming. (Bonus points if the word is also useful as a part of function names while programming.) This question is very well straddling the line between here and StackOverflow, but its more about English than programming, I think.

So, does the word I want exist?

  • 3
    I believe in your 1st two examples you are mixing meaning of sequential/concurrent and blocking/non-blocking which I find more appropriate in this context. If you consider this, sequentially fits in the other examples.
    – luk32
    Commented Aug 16, 2013 at 17:30
  • 1
    A common word for running a procedure which the system waits for is running it synchronously.
    – MetaEd
    Commented Aug 16, 2013 at 17:38
  • 1
    The word sequentially implies some of the same things as concurrently. Which things??
    – Hot Licks
    Commented Nov 14, 2015 at 2:30

6 Answers 6


I don't think the word you want exists in common usage. I suspect that's because the image you're trying to conjure is (as you describe) not merely "synchronously" but "synchronously when asynchronously is assumed".

In other words, you're trying to capture the idea that, though the reader expects that this procedure can be executed in parallel, it in fact cannot, and blocks program execution until it is done. That "blockingness" is essential to what you want to communicate, and I suspect that because it is fairly new to expect things to be done in parallel, there's no good common word available.


The word used in the field of Computing and Information Science is serially.

In the analysis of algorithms, a fundamental principle is that the parallelization of an algorithm cannot reduce the required time below the serial component of the calculation, even with an infinity of available processors, memory, etc.


I believe you're looking for sequentially

: of, relating to, or arranged in a sequence : serial

: following in sequence

: relating to or based on a method of testing a statistical hypothesis that involves examination of a sequence of samples for each of which the decision is made to accept or reject the hypothesis or to continue sampling


Thinking more on it, I'm not sure that they're opposite's per say, but a process is either sequential or parallel. I would consider an opposite to be a direct opposition to whatever it's being compared to, like backwards or forwards. In this case I think it's just one of two choices.

  • There is also seriatim, but I think it means about the same thing as sequentially in this context.
    – bib
    Commented Aug 16, 2013 at 18:03
  • Aye, on a conceptual level, sequentially sorta works. But you can't use it the same way as concurrently, because it is meaningless to do something "sequentially with" another thing. "in sequence with" works, but doesn't mean precisely the opposite. Part of the difficulty is that "in sequence" really wants to carry information about which thing happens first. "In parallel" doesn't carry that information because it doesn't need to.
    – alficles
    Commented Aug 16, 2013 at 18:03
  • @alficles: That's not true. In a program every command is a sequential step of an algorithm. If you sequential computer a set of values they don't necessarily have anything to do with each other, one is simply done after the other.
    – Jacobm001
    Commented Aug 16, 2013 at 18:44

The word "sequential" is used as an antonym for both "concurrent" and "parallel"; when these are explicitly distinguished, concurrent/sequential and parallel/serial are used as opposing pairs.

Patterson & Hennessy 2013, p. 503.


Could you not use the word discordantly if you do not want to connote the sequentiality of sequential or serial.


not agreeing, not in harmony, at variance.


I would use the word "Atomic" or "Atomicly

  • But even in the software world, atomically means indivisible, it is not the opposite of parallel.
    – Chenmunka
    Commented Nov 16, 2015 at 13:32
  • 1
    Right. Atomically would work in some circumstances. In some cases, the sequentialness is caused by atomicity. But in other cases, non-atomic things are run sequentially due to oversight, error, hardware limitation, or many other things. In these cases, the tasks may be non-atomic, but still sequential. It's definitely a good word to have in the lexicon of task scheduling, but it's not a 100% answer.
    – alficles
    Commented Nov 17, 2015 at 0:05

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