unpredictable

/ˈʌnprəˌdɪktəbəl/

/ənprɪˈdɪktəbəl/

When something's unpredictable, it doesn't happen at expected times or it can't be predicted. If trains were unpredictable, they wouldn't follow a set schedule and you'd never know when one would arrive.

The adjective unpredictable can also describe something that's unexpected or not known in advance. Your soccer team's skill level might be wildly unpredictable — from game to game your coach never knows what you guys will do. It can be an insult if someone calls you an unpredictable person. That means people can't rely on you for things, such as consistently good work or keeping your promises.

Definitions of unpredictable

1. adjective

   unknown in advance
   “an unpredictable (or indeterminable) future”

   synonyms:

   indeterminable, undeterminable

   not capable of being definitely decided or ascertained
2. adjective

   not occurring at expected times
   synonyms: irregular

   sporadic

   recurring in scattered and irregular or unpredictable instances
3. adjective

   not capable of being foretold
   synonyms:

   uncertain

   not certain to occur; not inevitable

   aleatory

   dependent on chance

   capricious, freakish

   changeable

   episodic, occasional

   occurring or appearing at usually irregular intervals

   unforeseeable

   incapable of being anticipated

   antonyms:

   predictable

   capable of being foretold

   certain, sure

   certain to occur; destined or inevitable

   foreseeable

   capable of being anticipated

   inevitable

   invariably occurring or appearing