A compass is a tool which can be used for drawing circles and arcs.
Compasses typically consists of two legs, one with a sharp spike at its end and the other with a pencil or pen attached to it. The purpose of the sharp spike is to fix the position of the leg. The other leg can then be rotated around that position and with the pencil an arc or a circle can be drawn.
The legs of most compasses can be locked so that the distance between the sharp spike and the pen remain unchanged. This enables the user to make several circles with the same radius or put a mark at a given distance from a point. The latter can for example be used to draw an equilateral triangle.