From my experience, it seems that although unstable is more commonly used, instable is often preferred in engineering and scientific contexts, e.g. "aircraft instability", "instable algorithm".

Are there any differences in the implied meaning of the two terms? Should unstable be preferred?


4 Answers 4


I have not seen the word "instable" being used often. The word "instability" exists, though. Funnily, the word "unstability" does not exist.

And even in engineering, the same two words are used - "unstable sorting algorithm", "unstable equilibrium", and as you said, "aircraft instability".

Instability is just the noun form of unstable.

EDIT from comments: The word "unstability" does exist, apparently, but is rarely used. I personally have never seen it. Even the spell check in Firefox marks it as a spelling mistake and suggests "instability" instead.

  • 7
    Wiktionary und Merriam Webster do have definitions for unstability, even though they do not provide much content: just declare the word as rare and link to instability.
    – Em1
    Commented Jul 30, 2012 at 7:44
  • Funnily? Why not funny enough? Commented Jul 30, 2012 at 15:52
  • 2
    Funnily enough, you mean? Commented Jul 30, 2012 at 17:50
  • 4
    That wouldn't be funny enough.
    – MetaEd
    Commented Jul 30, 2012 at 18:14
  • @MετάEd, @​asymptotically. Well, maybe it's because someone in the past had decided to make the noun form of "unstable" a bit unstable ;)
    – Pacerier
    Commented Nov 9, 2015 at 20:32

As asymptotically says, the adjective takes un-, the noun, in- (unstable, instability). A few other adjective/noun pairs behave this way:

  • unable, inability
  • unequal, inequality
  • ungrateful, ingratitude

So far as I’m aware, only Latinate roots show this alternation (hence, Germanic unhappy, unhappiness), but by no means all do (witness inefficacious, inefficacy; irrelevant, irrelevance). I don’t know what distinguishes those that do from those that don’t. I suspect historical accident, but I’d be interested to hear from those in the know.

  • 3
    Ask and it will be given to you; seek and you will find.
    – RegDwigнt
    Commented Jul 30, 2012 at 11:11
  • Ah, excellent, so it is historical accident: we only borrowed inequality, not “inequal” (inégal). Thanks for the link. Commented Jul 30, 2012 at 12:37
  • @Daniel, What's the difference between "unable" and "inable"?
    – Pacerier
    Commented Jan 21, 2014 at 8:20
  • The principle difference is that unable is a common word of English, but inable is not. Conversely, we have inability but not unability. That's the point of the Nathaniel's question. Commented Jan 22, 2014 at 5:59

Unstable seems to be in common use, but I have not often run across instable. In mathematics, engineering, electronics, and aeronautics I have often seen and used astable. As in "A rocket is astable." An astable multivibrator. Nevertheless, I've not seen astability, but have used instability. I'm not aware of a difference in meaning between un/in/astable.

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    I've seen astable used only for circuits. Here, 'a' is shown to mean 'zero' (rather than 'not'), meaning it is not stable in any state - as opposed to monostable and bistable. Commented Jul 30, 2012 at 9:38

I was looking for an answer myself, and looking around on the net, my conclusion is that something is unstable when it is in the state of instability.

Sometimes people mix them, presumably not knowing better. And as it happens with languages, such errors become more and more accepted, finally finding their way into the dictionary.

Being an electronics engineer myself, I confirm that the terms are sometimes mixed, although no specific meaning is associated with it (but it may appear as if you have more insight when you say instable instead of the more mundane unstable).

My reason for looking up the difference was because I saw the term "instable air" in a meteorological report. Again, my conclusion is the same — i.e. it sounds finer.

As for the term astable mentioned in a comment, in electronics it is used as follows:

  • monostable: something that has one stable state, i.e. it can be triggered from the stable state to the unstable state, from which it will automatically return after a certain period (hence unstable).
  • bistable: something that has two stable states, i.e. it can be triggered from one stable state to the other and vice versa.
  • astable: something that has no stable states, i.e. it will continuosly shift between the two states, both being unstable, i.e. it will stay in each of the states for a certain period.

You may then say that astable is something that is intentionally unstable.

Using the term "astable rocket" is to my believing wrong. A rocket is by nature unstable and therefore difficult to steer.

By the way, my native language is Danish, so excuse me for interfering in the English discussion.

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