Below are 2 different ways to define diameter of a circle.

**Definition**: The diameter of a circle is the distance from one edge of the circle to the other edge **and**, most importantly, through the circle's center.

or

**Definition**: The diameter is the length of the segment through the center that touches two points on the edge of the circle.

Both of the above definitions mean the same thing, but sometimes the easiest way to understand something is through a picture. Below is a diagram of the diameter

No matter how the line on the left rotates, it is always the circle's diameter because the line extends from one edge of the circle to the other **and** the line goes through the circle's center.