penumbra

puh-NUM-bra

meaning in plain English

Imagine the fuzzy edge of a shadow when the sun is only partly covered; that faint, partly lit ring is called a penumbra.

Picture a penumbra as the blurry edge of a shadow, like the gentle halo around a flashlight’s beam.

for example

When the moon slipped in front of the sun, the penumbra fell over the park, making everything look a shade dimmer than usual.

Think of a penumbra like the soft border around a candle’s glow when you dim the lights. It’s not completely dark, but not fully bright either—just a middle‑gray zone.