What is 

1. radius
2. diameter
3. circum fence
4. chord?

The definition of radius, diameter and chord are well explained in the revision notes. 

Circumference: It is the distance around a circle.

You can learn more about these by going through the following link:

https://www.meritnation.com/ask-answer/question/what-is-radius-diameter-chord-segment-etc-pls-tell-me/understanding-elementary-shapes/1474105