water joint

from the GNU version of the Collaborative International Dictionary of English.

• (Arch.) A joint in a stone pavement where the stones are left slightly higher than elsewhere, the rest of the surface being sunken or dished. The raised surface is intended to prevent the settling of water in the joints.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun architecture A joint in a stone pavement where the stones are left slightly higher than elsewhere, the rest of the surface being sunken, intended to prevent the settling of water in the joints.