A linear dimension is a measure of length that is used to measure the distance between two points. It is also known as a straight line. A radial dimension, on the other hand, is a measure of distance that is used to measure the distance from the center point of an object.
The difference between these two dimensions can be seen in this diagram:
In this diagram, you can see that the blue line represents a linear dimension while the green circle represents a radial dimension. The green circle has a radius of 10 centimeters while the blue line has a length of 20 centimeters.