- aqueduct: A water-conduit, particularly one for supplying a community from a distance.
- aqueous: Of, pertaining to, or containing water.
- arbiter: One chosen or appointed, by mutual consent of parties in dispute, to decide matters.
- arbitrary: Fixed or done capriciously.
- arbitrate: To act or give judgment as umpire.
- arbor: A tree.
- arboreal: Of or pertaining to a tree or trees.
- arborescent: Having the nature of a tree.
- arboretum: A botanical garden or place devoted to the cultivation of trees or shrubs.
- arboriculture: The cultivation of trees or shrubs.
- arcade: A vaulted passageway or street; a roofed passageway having shops, etc., opening from it.
- archaeology: The branch of anthropology concerned with the systematic investigation of the relics of man.
- archaic: Antiquated
- archaism: Obsolescence.
- archangel: An angel of high rank.
- archbishop: The chief of the bishops of an ecclesiastical province in the Greek, Roman, and Anglican church.
- archdeacon: A high official administrator of the affairs of a diocese.
- archetype: A prototype.
- archipelago: Any large body of water studded with islands, or the islands collectively themselves.
- ardent: Burning with passion.
- ardor: Intensity of passion or affection.
- arid: Very dry.
- aristocracy: A hereditary nobility
- aristocrat: A hereditary noble or one nearly connected with nobility.
- armada: A fleet of war-vessels.
- armful: As much as can be held in the arm or arms.
- armory: An arsenal.
- aroma: An agreeable odor.
- arraign: To call into court, as a person indicted for crime, and demand whether he pleads guilty or not.
- arrange: To put in definite or proper order.
- arrangement: The act of putting in proper order, or the state of being put in order.
- arrant: Notoriously bad.
- arrear: Something overdue and unpaid.
- arrival: A coming to stopping-place or destination.
- arrogant: Unduly or excessively proud, as of wealth, station, learning, etc.
- arrogate: To take, demand, or claim, especially presumptuously or without reasons or grounds.
- Artesian well: A very deep bored well. water rises due to underground pressure
- artful: Characterized by craft or cunning.
- Arthurian: Pertaining to King Arthur, the real or legendary hero of British poetic story.
- artifice: Trickery.
- artless: Ingenuous.
- ascendant: Dominant.
- ascension: The act of rising.
- ascent: A rising, soaring, or climbing.
- ascetic: Given to severe self-denial and practicing excessive abstinence and devotion.
- ascribe: To assign as a quality or attribute.
- asexual: Having no distinct sexual organs.
- ashen: Pale.
- askance: With a side or indirect glance or meaning.
- asperity: Harshness or roughness of temper.