An arc is a curve. You can describe the bend of a rainbow as an arc.
A portion of a curved line; 
Noun
 electrical conduction through a gas in an applied electric field
Noun
 a continuous portion of a circle
Noun
 something curved in shape
Verb
 form an arch or curve; "her back arches"; "her hips curve nicely"
n.
 A portion of a curved line; as, the arc of a circle or of an
   ellipse.
n.
 A curvature in the shape of a circular arc or an arch; as, the
   colored arc (the rainbow); the arc of Hadley's quadrant.
n.
 An arch.
n.
 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.
Arc
Statues and trophies, and triumphal arcs.4.
A portion of a curved line; 
Usage Examples
The arc of American history almost inevitably moves toward freedom. Whether it's Lincoln and the Emancipation Proclamation, the expansion of women's rights or, now, gay rights, I think there is an almost-inevitable march toward greater civil liberties.
Misspelled FormArc, Arc, rc, Arc, Aerc, A4rc, A5rc, Atrc, Afrc, Aec, A4c, A5c, Atc, Afc, Arec, Ar4c, Ar5c, Artc, Arfc, Arxc, Ardc, Arfc, Arvc, Ar c, Arx, Ard, Arf, Arv, Ar , Arcx, Arcd, Arcf, Arcv, Arc .
Other Usage Examples