penumbra
- The partially shaded outer region of a shadow cast by an opaque object, especially the area of partial eclipse surrounding a total eclipse.
- Figuratively, a peripheral or uncertain area surrounding a definite thing.
In a sentence
- Observers in the penumbra of the eclipse saw only a partial dimming of the sun rather than total darkness.
- The legal scholar argued that the right to privacy existed in the penumbra of the Bill of Rights.
Usage note. The 'umbra' is the total shadow at the center; the 'penumbra' is the partial shadow around it—remember 'paene' means 'almost', so penumbra is 'almost a shadow'.
Where it comes from
English penumbra — “partial eclipse shadow” · entered English 17th c.
A Modern Latin coinage formed from the Latin prefix 'paene' meaning 'almost' and 'umbra' meaning 'shadow'. The term was coined by the astronomer Johannes Kepler in the early 17th century in his astronomical writings. It entered English in the 17th century in the context of optics and astronomy.