umbra

noun

1. The fully shaded inner region of a shadow cast by an opaque object.
2. The fully shaded inner region of a shadow cast by an opaque object.
3. The central region of a sunspot.
4. (literary)A shadow.
5. (archaic)An uninvited guest brought along by one who was invited.
6. One of the family Umbridae of mudminnows.
7. One of genus Umbrina of drums (family Sciaenidae).
8. An element of the umbral calculus.

   “In order to set up such an algorithm, we need the notion of multiplicative inverse of an umbra. Two umbrae are said to be multiplicative inverse to each other when αγ ≡ u.”