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plural umbras or umbrae or (obsolete) umbræ
The fully shaded inner region of a shadow cast by an opaque object.
(astronomy) The area on the earth or moon experiencing the total phase of an eclipse. examples
(astronomy) The central region of a sunspot. examples
(chiefly literary) A shadow. examples
(archaic) An uninvited guest brought along by one who was invited.
One of the family Umbridae of mudminnows. examples
One of genus Umbrina of drums (family Sciaenidae). examples
(mathematics) An element of the umbral calculus. quotations examples
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)#