The Collaborative International Dictionary
Water joint
Water joint \Wa"ter joint`\ (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.
Wiktionary
water joint
n. (context architecture English) 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.