A different sense of the word "umbra", used in mathematics, is explained in the article titled umbral calculus.

The umbra (Latin for shadow) is the darkest part of a shadow. From within the umbra, the source of light is completely blocked by the object causing the shadow. This contrasts with the penumbra where the light source is not completely blocked and there is only a partial shadow.

The umbra is also the comparatively dark central region of a sunspot.

In the World of Darkness role-playing game setting, the term Umbra is the most commonly used term for those realms beyond the reaches of physical reality, such as the spirit world, the afterlife, and the astral plane.

See also eclipse.

The word is in the famous Latin phrase Pulvis et umbra sumus (dust and shadow we are) from Horace (Quintus Horatius Flaccus), Carmina (Book IV, 7, 16)