means The partial, fuzzy outer region of a shadow, where the light source is only partly blocked rather than fully obscured.

from From Latin paene 'almost' (also lurking in 'peninsula,' an almost-island) joined to umbra 'shadow' — literally an 'almost-shadow.' The astronomer Johannes Kepler is credited with coining it in the early 17th century to describe the gentler gray fringe around the dark heart of an eclipse's shadow.