adj.
1 capricious, varying, erratic, uncertain, inconsistent, doubtful, unpredictable, whimsical, irrational, chance, random, inconsistent, subjective, unreasoned, irrational, Colloq chancy, iffy:
The choices are entirely arbitrary, totally at the whim of the council and not based on research or knowledge.
2 absolute, tyrannical, despotic, authoritarian, magisterial, summary, peremptory, autocratic, dogmatic, imperious, uncompromising, inconsiderate, high-handed, dictatorial, Rare thetic(al):
The conduct of the archbishop appears to have been arbitrary and harsh.