English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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?

share|improve this question
Coming from a CS perspective, I've never heard of an "instable algorithm" before. "Unstable algorithms" (such as an "unstable sort") are common, though. – Izkata Jul 30 '12 at 13:48
You may also see englishforums.com/English/InstableVsUnstable/xmdjh/post.htm – Stat-R Jul 30 '12 at 17:31
up vote 25 down vote accepted

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.

share|improve this answer
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 Jul 30 '12 at 7:44
Funnily? Why not funny enough? – Chibueze Opata Jul 30 '12 at 15:52
Funnily enough, you mean? – asymptotically Jul 30 '12 at 17:50
That wouldn't be funny enough. – MετάEd Jul 30 '12 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 Nov 9 '15 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.

share|improve this answer
Ask and it will be given to you; seek and you will find. – RegDwigнt Jul 30 '12 at 11:11
Ah, excellent, so it is historical accident: we only borrowed inequality, not “inequal” (inégal). Thanks for the link. – Daniel Harbour Jul 30 '12 at 12:37
@Daniel, What's the difference between "unable" and "inable"? – Pacerier Jan 21 '14 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. – Daniel Harbour Jan 22 '14 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.

share|improve this answer
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. – asymptotically Jul 30 '12 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.

share|improve this answer

protected by RegDwigнt Nov 26 '13 at 20:55

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site.

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.