Had to dig deep this week for a "U":
Umbra: the darkest part of a shadow. It is actually the Latin word for shadow. You usually only see it used in relation to lunar eclipses (combined with prenumbra and antumbra).
So in the below photo of the southern wall of the Castillo de San Marcos, the left hand portion is an umbra because it is in full shadow... I think... ah, who cares. It's a "U", it's a photo. Mission accomplished. :)