obscuration

noun

1. (countable, uncountable)The state of being obscured.

   “Money is miraculous. What miraculous facilities has it yielded, will it yield us; but also what never-imagined confusions, obscurations has it brought in; down almost to total extinction of the moral-sense in large masses of mankind!”

   “At eleven the room darkened, the moon having climbed behind a tree. But the tree being small, and the moon's ascension rapid, this transit was brief, and this obscuration.”

2. (countable, uncountable)A unit of measurement used in particular for smoke detectors which respond to absorption of light by smoke, in percent absorption per unit length, e.g. % obs/ft, % obs/m.