a.

1. 隨心所欲的，武斷的
   This is an arbitrary decision.

   這是個武斷的決定。

2. 反覆無常的，任性多變的
   She married an artist with an arbitrary character.

   她嫁給一個性格反覆無常的藝術家。

3. 獨斷的，專制的
   Dictators are arbitrary rulers.

   獨裁者即獨斷專橫的統治者。

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.