A Cross-cutting relationship is the principle in geology concerning crossing layers.
The photo shows a really good example of cross-cutting relations. The light gray intrusion is located in Sweden. It is cut by a white intrusion (pegmatite), which is cut by a black intrusion (diabase). Since the black intrusion is whole (not cut by any intrusion) it is the youngest. The white intrusion is the second youngest because it is cut by the black intrusion, but cuts in to the light-gray intrusion. The light-gray intrusion is the oldest because: it is cross-cut by the white and the black intrusion.