define a reference point?

A refrence point is a point in space with respect to which the position of an object in space is expressed.

It is a fixed point and does not move so that the position of an object is described by the coordinate with respect to the reference point. In three dimensional coordinate system, the point of intersection of three axes(x, y and z axis) i.e Origin is considered as the reference point. The position of an object is taken as the distance of the object from the origin. In 3-d space, the origin act as the reference point.