"Instable" or "unstable"?

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?

Solution 1:

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.

Solution 2:

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.