umbra

plural umbras · plural umbrae · obsolete plural umbræ

1. The fully shaded inner region of a shadow cast by an opaque object.
   1. astronomyThe area on the earth or moon experiencing the total phase of an eclipse.
2. astronomyThe central region of a sunspot.
3. literaryA shadow.
4. archaicAn uninvited guest brought along by one who was invited.
5. One of the family Umbridae of mudminnows.
6. One of genus Umbrina of drums (family Sciaenidae).
7. mathematicsAn 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.

   2009, Ernesto Damiani, Ottavio D'Antona, Vincenzo Marra, From Combinatorics to Philosophy: The Legacy of G.-C. Rota, (page 113)#