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arc·length | \ (ˌ)ärk-ˈleŋ(k)th 
, -ˈlen(t)th \
, -ˈlen(t)th \ plural arclengths
Definition of arclength
geometry
: the length of a segment of an arc
Thus, in a unit circle (radius = 1), any angle of 1 radian cuts off an arc of unit length. All other arclengths are proportional to this; for instance, doubling the radius (to 2) and tripling the angle (to 3 radians) yields an arclength of 6.— Rheta N. Rubenstein et al., Math Horizons, November 2003
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The first known use of arclength was in 1849
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