A half shadow. The partially shaded outer region of the shadow cast by an opaque object. Also, A peripheral or indeterminate area or group.
Mid 17th century: modern Latin, from Latin paene ‘almost’ + umbra ‘shadow’.
“As I dodged the heat of the day, standing in the penumbra of the large mansion house.”
“The problem is the penumbra of theory expands the potential answer too greatly.”