ar·bi·trar·y ˈärbəˌtrerē/ adjective adjective: arbitrary based on random choice or personal whim, rather than any reason or system. "his mealtimes were entirely arbitrary" synonyms: capricious, whimsical, random, chance, unpredictable; More antonyms: reasoned, rational (of power or a ruling body) unrestrained and autocratic in the use of authority. "arbitrary rule by King and bishops has been made impossible" synonyms: autocratic, dictatorial, autarchic, undemocratic, despotic, tyrannical, authoritarian, high-handed; More antonyms: democratic MATHEMATICS (of a constant or other quantity) of unspecified value. Origin late Middle English (in the sense ‘dependent on one's will or pleasure, discretionary’): from Latin arbitrarius, from arbiter ‘judge, supreme ruler,’ perhaps influenced by French arbitraire .