1. the partially shaded outer region of the shadow cast by an opaque object
Usage: astronomy; physics
2. a surrounding area or fringe where something exists to a lesser degree
Usage: formal

Examples

• During the eclipse, observers in the penumbra saw only a partial darkening of the sun.
• The moon’s penumbra created a ring of partial shadow on Earth’s surface.
• The city existed in the penumbra between urban development and rural farmland.
• Legal scholars debated the penumbra of rights implied by the Constitution.
• The forest’s penumbra gradually gave way to open meadow.
• Scientists studied the penumbra surrounding the black hole.
• The artist captured the subtle gradations within the shadow’s penumbra.