Arbitrary is a word that means something done without any specific reason or support. The opposite of the word arbitrary could either be predictable or logical. Predictable could mean following a set pattern or certain expectations, which is the opposite of arbitrary, which means going against the norm, and doing something out of the blue. Logical, on the other hand, suggests that an action is carried out based on sound reasoning or a set of rational standards or principles. The antonyms of arbitrary, predictable and logical, could be used in different contexts to refer to actions or decisions that were carried out based on the specific demands or requirements of a situation.