What are the difference's between contour interval and index contour?

the contour interval is the difference in elevation between contour lines that are next to each other

A contour interval shows a change in elevation on a topographical map. If the contour interval is 20 then the elevation change between two contour lines is 20 meters

The difference in elevation between two contour lines that are side by side is the contour interval. The contour interval is the vertical distance between two adjacent contour lines on a topographic map.

Notice the pattern around that contour line. Then determine the interval that the surrounding contour lines are increasing or decreasing by.

Ex. 50 100 150 200, the contour interval would then be 50