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 ...

2011年4月6日 · Nouns in "-ity" don't always take the same negative prefix as the corresponding adjective. For example, "instability" is greatly preferred over "unstability", but "unstable" is greatly preferred over "instable". –

2011年3月10日 · The prefixes in-and un-both have the effect of negating the meaning of the word. The prefix in-comes from Latin and almost exclusively applies to words borrowed from Latin, which in many cases were borrowed from Latin with the in-prefix already attached.