Hyperuniform materials are mixed-component many-particle systems with unusually low fluctuations in component density at large scales, when compared to the distribution of constituents in common disordered systems, like a mixed ideal gas (air) or typical liquids or amorphous solids: A disordered hyperuniform system is statistically isotropic, like a liquid, but exhibits reduced long-wavelength density fluctuations, similar to crystals. All perfect crystals, perfect quasicrystals and special disordered systems are hyperuniform.