What is the elevation change between two adjacent lines on a map?

Those lines representing elevation on a map are called contour lines. The difference in elevation between two of these lines is called the contour interval. Different maps use different contour intervals based on the scale of the map, or in other Words, the size of the contour interval is based on how zoomed in and detailed the map is.

Sometimes a map will have darker and thicker contour intervals. This is called the Index Contour Interval. Index contour intervals appear less frequently and represent a larger elevation change. It helps you figure out the amount of a large elevation difference faster because usually they are multiples of 100 or 1000, making them easier to add up.