A design, usually in the background, mimicking the contour lines of topographic maps used to delineate altitude changes.

The lines on topogrqphic maps.
"What is a Topographic Map? USGS":[https://www.usgs.gov/faqs/what-a-topographic-map#:~:text=The%20distinctive%20characteristic%20of%20a%20topographic%20map%20is,reference%20surface%2C%20which%20is%20usually%20mean%20sea%20level.]
elevation contour lines