Subsurface scattering
Light entering a surface, spreading underneath it, then coming back out. It is the soft glow in skin, wax, marble, and leaves.
See it
What it is
Subsurface scattering happens when light enters a material, bounces around below the surface, and exits somewhere else. That travel softens small shadows and lets thin areas glow. Skin shows it in a backlit ear, where red wavelengths travel farther than blue; wax, milk, marble, and leaves each scatter light at their own scale and color.
Reach for it when an otherwise correct material still reads as hard plastic. Offline renderers approximate it with the dipole model Henrik Wann Jensen published in 2001; real-time work usually settles for a thickness map, a blurred lighting pass, or a backlighting term to capture the recognition cues.
Gotcha: this is not transparency and it is not emission. The object should not become uniformly see-through or glow in darkness. Scene scale also matters because scattering distance is a physical length: settings that suit a millimeter-thin leaf will turn a life-size marble slab into fog.
Ask AI for it
Build this skin material in Blender Cycles with the Principled BSDF. Raise Subsurface Weight from zero, use Subsurface Radius to let red travel farther than green and blue, and keep Transmission at zero so the head does not become glass. Drive the amount with a thickness map so ears and nostrils scatter more than the forehead, then test it with a strong area light behind the subject.