pe · num · bra [New Latin penumbra : Latin paene, 'almost' + Latin umbra, 'shadow'.] n. pl. pe·num·brae or pe·num· bras; adj. pe·numbral or pe·numbrous

  1. An area within which distinction or resolution is difficult or uncertain.
  2. An outlying surrounding region; a periphery.
  3. 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).]
  4. A blurred area in a radiograph at the edge of an anatomical structure.
  5. A partial shadow, as in an eclipse, between regions of complete shadow and complete illumination.
  6. An area in which something exists to a lesser or uncertain degree.
  7. The part of a picture where the shade imperceptibly blends with the light.