a.

1. 随心所欲的，武断的
   This is an arbitrary decision.

   这是个武断的决定。

2. 反复无常的，任性多变的
   She married an artist with an arbitrary character.

   她嫁给一个性格反复无常的艺术家。

3. 独断的，专制的
   Dictators are arbitrary rulers.

   独裁者即独断专横的统治者。

adj.

1. based on random choice or personal whim.
2. (of power or a ruling body) autocratic.
3. Mathematics  (of a constant or other quantity) of unspecified value.