A nodal-line semimetal has band crossings that form a closed loop (nodal ring) in the 3D Brillouin zone, rather than isolated Weyl points. The ring is protected by mirror or PT symmetry.
Projected onto a surface, the nodal ring becomes a region bounded by the projected loop. Inside this region, a drumhead surface state appears — a nearly flat band at zero energy (like the membrane of a drum).
Examples: Ca₃P₂, Cu₃PdN, ZrSiS. The flat band has enormous DOS, potentially leading to exotic magnetism or superconductivity.