1. The invariable or characteristic accompaniment or companion of a person or thing.
2. Shade; shadow.
3. In astronomy, the complete shadow of an opaque body, as a planet, where the direct light from the source of illumination is completely cut off.
4. A phantom or shadowy apparition, as of someone or something not physically present; ghost; spectral image.
She leaned over me, dark and mother-warm, and I could hardly breathe in her umbra of cigarettes and gin.
-- Jonathan Strahan, Eidolon
The umbra - shade, shadow, uninvited guest - is invited in; as a dream, the guest becomes host to a sensuous pleasure that is all the more real for being an imitation, and all the more an artifice for being imaginal, "only" a dream.Umbra is borrowed from the same word in Latin, meaning "shadow."
-- Patricia Cox Miller, Dreams in Late Antiquity: Studies in the Imagination of a Culture