penumbra

noun

pen·​um·​bra pə-ˈnəm-brə 

plural penumbrae pə-ˈnəm-(ˌ)brē 

-ˌbrī

or penumbras

1

a

: a space of partial illumination (as in an eclipse) between the perfect shadow on all sides and the full light

b

: a shaded region surrounding the dark central portion of a sunspot

2

: a surrounding or adjoining region in which something exists in a lesser degree : fringe

the seventeenth century lay in the penumbra of the middle ages

—Edward Eggleston

3

: a body of rights held to be guaranteed by implication in a civil constitution

the penumbra of the Bill of Rights

4

: something that covers, surrounds, or obscures : shroud

a penumbra of secrecy

a penumbra of somber dignity has descended over his reputation

—James Atlas

penumbral

pə-ˈnəm-brəl 

adjective

Did you know?

Every solar eclipse casts an umbra, the darker central area in which almost no light reaches the earth, and a penumbra, the area of partial shadow where part of the sun is still visible. Penumbra can thus be used to describe any "gray area" where things aren't all black and white. For example, the right to privacy falls under the penumbra of the U.S. Constitution; though it isn't specifically guaranteed there, the Supreme Court has held that it is implied, and thus that the government may not intrude into certain areas of a citizen's private life. Because its existence is still shadowy, however, the Court is still determining how much of an individual's life is protected by the right to privacy.

Synonyms

• shadow
• shade
• blackness
• dusk
• gloom
• umbra

Examples of penumbra in a Sentence

the lunar eclipse began with a subtle darkening of the lunar surface as it passed within the Earth's penumbra

a penumbra of despair fell over the doomed city

Recent Examples on the Web

During a partial solar eclipse, the umbra misses Earth, with only the penumbra — the moon's outer, fuzzier and lighter shadow — striking Earth. —Jamie Carter, Forbes.com, 18 Sep. 2025

The umbra is a darker inner shadow and the penumbra is a fainter outer shadow, according to NASA. —Melina Khan, USA Today, 6 Sep. 2025

The stages of the total lunar eclipse The moon enters Earth's penumbra The soft, outer part of Earth's shadow, known as the penumbra, will begin to sweep across the lunar surface at 11:28 a.m. EDT (1528 GMT) on Sept. 7. —Anthony Wood, Space.com, 6 Sep. 2025

The umbra is a darker inner shadow and the penumbra is a fainter outer shadow, according to NASA. —Marina Johnson, Louisville Courier Journal, 5 Sep. 2025

Word History

Etymology

New Latin, from Latin paene almost + umbra shadow — more at umbrage

First Known Use

1665, in the meaning defined at sense 1a

Rhymes for penumbra

• umbra

Kids Definition

penumbra

noun

pen·​um·​bra pə-ˈnəm-brə 

plural penumbrae -brē 

-ˌbrī

or penumbras

: the partial shadow surrounding a perfect shadow (as in an eclipse)

penumbral

-brəl 

adjective

Medical Definition

penumbra

noun

pe·​num·​bra pə-ˈnəm-brə 

plural penumbrae -(ˌ)brē  -ˌbrī  or penumbras

: a blurred area in a radiograph at the edge of an anatomical structure

penumbral

-brəl 

adjective

Legal Definition

penumbra

noun

pe·​num·​bra pi-ˈnəm-brə 

plural penumbras

1

: an area within which distinction or resolution is difficult or uncertain

the public-private penumbra

2

: an extension of protection, reach, application, or consideration

especially : a body of rights held to be guaranteed by implication from other rights explicitly enumerated in the U.S. Constitution

the First Amendment has a penumbra where privacy is protected from governmental intrusion

—Griswold v. Connecticut, 381 U.S. 479 (1965)

see also Griswold v. Connecticut

penumbral

-brəl 

adjective