Light entering a surface, bouncing around inside it and coming out somewhere else — the reason marble, skin, wax and milk look soft and glowing.
In translucent materials, light travels some distance beneath the surface before re-emerging, so each point's colour is blended with its surroundings and shadow edges soften. The depth depends on how strongly the material scatters and absorbs at each wavelength.
It is different from transparency: a subsurface-scattering material does not let you see through it, but it does let light through.
Marble looks luminous beside a painted wall of the same colour because light penetrates and scatters within it.
A flat colour sample cannot represent a translucent material, and measuring one needs care about the aperture and backing.
The effect is described by how far light travels inside a material before it re-emerges, which depends on how strongly the material scatters and absorbs at each wavelength. In computer graphics those two coefficients are measured by shining a narrow beam at a sample and recording the glow that spreads around the entry point. In colour measurement the same behaviour shows up as edge loss: light that enters under the instrument's aperture and leaves outside it is never collected, so a translucent sample reads darker through a small aperture than through a large one.
An engineered stone, a solid-surface worktop or a ceramic is colour-matched to real marble by measurement.
The numbers agree and the substitute looks flat and chalky beside the stone, because it returns its light from the surface instead of from depth. Translucency has to be specified as well as colour.
A 3D render of skin, wax, soap or milk uses a plain diffuse material.
Shadows have hard edges and thin parts such as ears do not glow when backlit, so the object reads as painted plaster. Surface-only shading models cannot reproduce these materials.