(i) A circle is a set of all those points in a plane, whose distance from a fixed point remains constant.
(ii) Radius of a circle is a line segment with one end at its centre and the other end on the circle. (It is the constant distance between all the points on the circle and its centre.)
(iii) The centre of a circle is that fixed point from which all points remain at a constant distance.
(iv) Diameter of a circle is a line segment passing through the centre of a circle, and having its end points on the circle.
(v) A chord of a circle is a line segment with its end points lying on the circle.
(vi) Interior of a circle is a set of all those points which lie inside the circle.