eclipse

(astronomy) An alignment of astronomical objects whereby one object comes between the observer (or notional observer) and another object, thus obscuring the latter.

Especially, an alignment whereby a planetary object (for example, the Moon) comes between the Sun and another planetary object (for example, the Earth), resulting in a shadow being cast by the middle planetary object onto the other planetary object.

(ornithology) A seasonal state of plumage in some birds, notably ducks, adopted temporarily after the breeding season and characterised by a dull and scruffy appearance.

Obscurity, decline, downfall.