The locus of points which are a fixed distance from [b]a given point[/b].[br][br]In plain terms:[br][br]This is a circle! Take a look at the graphic below. The [i]given point[/i] is the centre of the circle and the [i]locus [/i]are all of the points at a fixed distance from the centre.

The Centre of the circle is the given point in this example and the Circle represents the Locus. The Locus in this instance is equidistant from the given point. The measurement of this distance can be called the [i]Radius [/i]because that's exactly what it is!