Unpredictability is the quality of being impossible to know or guess in advance. When something has unpredictability, you can't reliably predict what will happen next, no matter how carefully you observe or plan.

Weather has a lot of unpredictability: even with satellites and computers, meteorologists sometimes get forecasts wrong because tiny changes in temperature or wind can create surprisingly different outcomes. A coin flip is often used as an example of unpredictability: there's no way to know whether it will land on heads or tails.

Unpredictability can make life exciting or frustrating, depending on the situation. The unpredictability of a mystery novel keeps you turning pages, eager to discover what happens next. But the unpredictability of a substitute teacher's rules might make it hard to know how to behave in class.

Some things seem unpredictable only because we don't understand them yet. Ancient people thought eclipses were unpredictable, but now we can calculate them centuries in advance. Other things, like exactly which card you'll draw from a shuffled deck, remain genuinely unpredictable no matter how much we learn.

The opposite of unpredictability is predictability: when something is so reliable and consistent that you always know what to expect.