The Collaborative International Dictionary of English v.0.48:

Arc \Arc\, n. [F. arc, L. arcus bow, arc. See Arch, n.]
   1. (Geom.) A portion of a curved line; as, the arc of a
      circle or of an ellipse.
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   2. A curvature in the shape of a circular arc or an arch; as,
      the colored arc (the rainbow); the arc of Hadley's
      quadrant.
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   3. An arch. [Obs.]
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            Statues and trophies, and triumphal arcs. --Milton.
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   4. The apparent arc described, above or below the horizon, by
      the sun or other celestial body. The diurnal arc is
      described during the daytime, the nocturnal arc during the
      night.
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   Electric arc, Voltaic arc. See under Voltaic.
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WordNet (r) 3.0 (2006):

electric arc
    n 1: electrical conduction through a gas in an applied electric
         field [syn: discharge, spark, arc, electric arc,
         electric discharge]