The quality of letting some light pass through, or being partially transparent, is translucence. The translucence of stained glass gives it a glowing quality when the sun shines on it.
When an object is completely clear, it's transparent, and when no light at all can pass through it, it's opaque. In between these two qualities is translucence; a translucent material, which lets some light through while not quite allowing a clear view of objects on the other side. The Latin roots of translucence are trans, "through," and lucere, "to shine."