A more thorough explanation:

Definition: A penumbra is an area or region that surrounds something, but its extent is uncertain or unclear. In constitutional law, the Supreme Court has ruled that the Bill of Rights contains implied rights, including the right to privacy, which are found in the penumbra of the Constitution.

Examples:

• The penumbra of a solar eclipse is the area of partial shadow surrounding the darkest part of the shadow.
• The penumbra of a city is the outskirts or suburbs that surround the main urban area.
• In literature, the penumbra of a character's personality refers to the hidden or ambiguous aspects of their character that are not fully revealed.

The examples illustrate the definition of penumbra by showing how it can refer to a variety of surrounding areas or peripheries that are not clearly defined or delimited. In each case, there is a central or core element (the darkest part of the shadow, the main urban area, the revealed aspects of a character's personality) that is surrounded by a less distinct or uncertain region (the partial shadow, the outskirts or suburbs, the hidden or ambiguous aspects of the character). The penumbra is a useful concept for describing these kinds of fuzzy or indeterminate boundaries.