arc (ärk)n.1. Something shaped like a curve or arch: the vivid arc of a rainbow.2. Mathematics A segment of a circle.3. An electric arc.4. Astronomy The apparent path of a celestial body as it rises above and falls below the horizon.5. A progression of events suggesting narrative cohesion, especially one that rises to a climax and settles to a final conclusion.intr.v. arced (ärkt), arc·ing (är?k?ng), arcs 1. To form an arc.2. To move or seem to move in a curved path: the stars that arc across the sky.[Middle English ark, from Old French arc, from Latin arcus.]ARCabbr.1. AIDS-related complex2. American Red CrossAmerican Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.