'Umbra' definitions:

Definition of 'umbra'

noun

A region of complete shadow resulting from total obstruction of light

From: GCIDE

Umbra \Um"bra\, n.; pl. Umbrae. [L., a shadow.]
1. (Astron.) (a) The conical shadow projected from a planet or satellite, on the side opposite to the sun, within which a spectator could see no portion of the sun's disk; -- used in contradistinction from penumbra. See Penumbra. (b) The central dark portion, or nucleus, of a sun spot. (c) The fainter part of a sun spot; -- now more commonly called penumbra. [1913 Webster]
2. (Zool.) Any one of several species of sciaenoid food fishes of the genus Umbrina, especially the Mediterranean species (Umbrina cirrhosa), which is highly esteemed as a market fish; -- called also ombre, and umbrine. [1913 Webster]
Umbra tree (Bot.), a tree (Phytolacca dioica) of the same genus as pokeweed. It is native of South America, but is now grown in southern Europe. It has large dark leaves, and a somber aspect. The juice of its berries is used for coloring wine. --J. Smith (Dict. Econ. Plants). [1913 Webster]

From: GCIDE

Penumbra \Pe*num"bra\, n. [NL., fr. L. paene almost + umbra shade.]
1. An incomplete or partial shadow. [1913 Webster]
2. (Astron.) The shadow cast, in an eclipse, where the light is partly, but not wholly, cut off by the intervening body; the space of partial illumination between the umbra, or perfect shadow, on all sides, and the full light. --Sir I. Newton. [1913 Webster]
Note: The faint shade surrounding the dark central portion of a solar spot is also called the penumbra, and sometimes umbra. [1913 Webster]
3. (Paint.) The part of a picture where the shade imperceptibly blends with the light. [1913 Webster]