What is the name for the darker contour lines on a topographical map?

The term for the darker contour lines on a topographical map is index contours. These lines are typically drawn at regular intervals and serve as a reference point to help map users understand the elevation and terrain more clearly. Index contours are generally thicker and more prominent than regular contour lines, which can be lighter and more frequent. By using these index contours, individuals can quickly assess elevation changes and identify significant features in the landscape, as they are marked with specific elevation values. This makes them crucial for effective navigation and understanding of topographical features on a map.

While key contours might relate to specific significant lines, and altitude lines suggest a level of elevation which can be seen in the context of contour lines, the standard terminology used in cartography for the bolded lines is indeed index contours. Gradient lines does not apply here as those would imply the steepness or degree of incline rather than a specific representation of elevation.