Umbra is a noun that refers to a shadow or darkness. 

Our word of the day began life as a Latin word for ‘shade.’ Since the late 16th century, it’s been an English word for ‘shade’ or ‘shadow.’ Here’s an example:

The long umbra cast along the lake is a sight to behold. The fact that it’s a shadow of a port-o-potty makes the image a little less romantic.