In the setting of the Rifts Roleplaying Game, Ley lines are areas of significant supernatural energy. They're bright enough to see at night, though not as much during the day. The places where they intersect are called nexus points, and those are places of particularly strong supernatural energy. Nexus points are where most of the rifts open up.

They represent a unique cartography challenge, since they're not really part of the terrain per se, but their locations are very important in the context of the setting.