Arbitrary means based on preference, notion, or whim.
Arbitrary is an adjective.
Synonyms: erratic, inconsistent, capricious, whimsical.
Antonyms: coherent, orderly.
The shopkeeper made an arbitrary decision to close the store four hours early so he could play with his children.
The lecturer's marks appear to be arbitrary; the students cannot see how they relate in any way to their performance.
There was no time to consider the matter, I just had to make an arbitrary decision.