What is the difference between instable and unstable?
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"instable" is not a word; the prefix "in-" is not correct for this word. The correct form for "not stable" is "unstable".
Then howcome both dictionary.com and oxford list it as meaning 'not stable'?
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Do you mean you saw "instable" in those dictionaries?? Or did you see the noun form, instability?

The adjective form of not stable is indeed unstable. But the noun form, not having stability, is instability.
I saw the adjective form as instable:

Hmmm. The World Engish Dictionary says it's 'less common' ... hell yes it's less common. In all my years (50+) I have NEVER seen or heard the word 'instable'. My advice is to forget that, at least in regard to American English. Just use unstable or instability.
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Newsflash, the rest of the world doesn't speak American english. Instable comes from the queens english which you americans need to adopt back into your hacked vocabulary

Forgive me for saying so, but that sounds a tad harsh..Emotion: crying

I bet the Queen is careful with capital letters and punctuation. Emotion: stick out tongue

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