Arc
From AoPSWiki
An arc of a circle is the portion of the circle between two given points on the circle. More generally, an arc is a portion of a smooth curve joining two points.
The measure of a circular arc 
on circle 
is defined to be the measure of the central angle 
which has the arc on its interior. The length of an arc can be calculated by the formula 
, where 
is the radius of the circle and 
is the measure of the arc, in radians. Thus, in particular, the circumference of a circle is given by 
.
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Alternate usage
Arc is also used as a prefix. For each of the standard trigonometric functions, the arc-function is (one of) the corresponding inverse function. For example, the arcsine is the inverse of the sine.
