umbra

umbra, that part of a shadow in which all light from a given source is excluded. The shadow from a point source of illumination is essentially all umbra, but that from a source of some size (as from the Sun) consists of a small umbra and a much larger partial shadow called the penumbra. Thus, in an eclipse of the Sun, the regions within the umbra experience a total eclipse and those within the penumbra, partial eclipse. The term is also used for the dark central portion of a sunspot. See also solar eclipse; lunar eclipse.