Geometric Definition:

An arc is a geometric shape used for drawing circle segments. It is comprised of three main parts:

• staring θ: the angle measure, for the first part of the sub-circle, usually in radians
• ending θ: the angle measure, for the ending part of the sub-circle, usually in radians
• radius:     the radius of the circle that this arc is a part of.

Arcs in computer programming cannot be accurately represented, as the value of π is infinite. Thus, any circle or arc you see will always be slightly inaccurate, although most algorithms usually keep that with a few pixels.

The length of an arc can be approximated by the following formula:

Notice that on some systems, the length of an arc can be negative, specifying that it moves counter-clockwise around the circle, instead of clockwise. This is usually represented by a boolean flag being set from the calling function.

On Specific Systems:

For windows, arcs can be drawn using the gdi+ functions, and can also draw arcs of ellipses.

Example in c#:

void drawArc(Graphics myGraphics, Pen myPen, float x, float y, float startTheta, float endTheta, float radius)
{
    myGraphics.DrawArc(myPen, x - radius, y - radius, radius * 2, radius * 2, startTheta, endTheta);
}

Arcs can also be drawing using the equivalent c++ drawing functions.

For Mac OS X, arcs can be drawn using the NSBezierPath class in AppKit.

Example in objective-c:

void drawArc(float x, float y, float startTheta, float endTheta, float radius)
{
    NSBezierPath *bezierPath = [NSBezierPath bezierPath];
    [bezierPath appendBezierPathWithArcWithCenter:CGPointMake(x, y) radius:radius startAngle:startTheta endAngle:endTheta];
    [bezierPath stroke]; // draws with current set color
}

In Java, you can draw arcs as well, using the portable functions found in the Graphics class.