Umbra \UHM-bruh\ , noun;
1. The invariable or characteristic accompaniment or companion of a person or thing
2. Shade; shadow
3. 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.
Based on the last definition, this is a probably a good word to file away in the memory for crossword puzzles...
This word, with the definition "phantom, ghost," dates to the 1590's from Latin umbra ("shade, shadow") which derives from Proto-Indo-European *andho- ("blind, dark"). The astronomy usage is first attested in the 1670's and the definition "an uninvited guest accompanying an invited one" dates tot he 1690's.
Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Friday, June 10
Etymologies come from the Oxford English Dictionary and/or Etymonline.com