The partially shaded outer region of a shadow cast by an opaque object — in astronomy, the area where only part of the light source is blocked.

From the Latin paene (almost, nearly) and umbra (shadow) — a near-shadow, the fringe region between full shadow and full light.

In legal scholarship, a penumbra refers to the implied rights or principles that surround and emanate from explicitly stated constitutional rights.

Famously invoked by Justice William O. Douglas in Griswold v. Connecticut (1965), where he argued that the right to marital privacy existed in the penumbra of the Bill of Rights.

Galileo used the word penumbra in his 1610 Sidereus Nuncius to describe the shadowed margins of craters on the Moon, making it one of the first scientific uses of the term.