A line from center to circumference of a circle is known as

A line from the centre to circumference of a circle is known as radius.

A straight line from one point on the circumference to the other point is known as diameter. Also, diameter $=2$ $\times$ radius

Area of circle is calculated by \pi$$r^2, where r is the radius and $\pi=\frac{22}{7}$

The midpoint of a circle is its centre.