The Definition of Penumbra

pe·num·bra /pɪˈnʌmbrə/ Pronunciation Key - Show Spelled Pronunciation[pi-nuhm-bruh] Pronunciation Key - Show IPA Pronunciation
–noun, plural -brae /-bri/ Pronunciation Key - Show Spelled Pronunciation[-bree] Pronunciation Key - Show IPA Pronunciation, -bras. 1. Astronomy. a. the partial or imperfect shadow outside the complete shadow of an opaque body, as a planet, where the light from the source of illumination is only partly cut off. Compare umbra (def. 3a).
b. the grayish marginal portion of a sunspot. Compare umbra (def. 3b).

2. a shadowy, indefinite, or marginal area.


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[Origin: 1660–70; < NL, equiv. to L paen- pen- + umbra shade]

—Related forms
pe·num·bral, pe·num·brous, adjective
Dictionary.com Unabridged (v 1.1)
Based on the Random House Unabridged Dictionary, © Random House, Inc. 2006.