Adventitious. Accidental, incidental, extrinsic, non-essential, foreign, casual; redundant, superfluous.
Ant. Intrinsic, pertinent, regular, proper, necessary, appropriate.
Adventure. Chance, hazard, risk, venture, experiment, trial; event, incident, occurrence, contingency, crisis.
Ant. Matter-of-fact, matter-of-course; prospect; monotony, routine.
Adventurous. Bold, daring, venturesome, rash, reckless, headlong; dangerous, perilous, uncertain; fearless, enterprising.
Ant. Timid, nervous, hesitating, cautious, cowardly.
Adversary. Foe, enemy, antagonist, opponent, rival, assailant.
Ant. Ally, helper, assistant, accomplice, aider, abettor, accessory.
Adverse. Opposite, contrary, conflicting; hostile, injurious, harmful; unlucky, unfortunate, disastrous.
Ant. Favorable, friendly, propitious, fortunate, lucky.