die Finsternis
darkness
A state of complete darkness in which there is no light or only very little light.
Als der Strom ausfiel, saßen wir in völliger Finsternis.
When the power went out, we sat in complete darkness.

darkness / eclipse
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darkness
A state of complete darkness in which there is no light or only very little light.
Als der Strom ausfiel, saßen wir in völliger Finsternis.
When the power went out, we sat in complete darkness.

'Die Finsternis' (darkness/eclipse) is a noun derived from the adjective 'finster' (dark, gloomy). The ending '-nis' forms an abstract noun here that describes the state of being dark.
Er hatte Angst vor der finsteren Gestalt im Schatten.
He was afraid of the dark figure in the shadow.
The word 'Finsternis' (darkness/eclipse) is almost always used in the singular. The plural 'die Finsternisse' is rare and refers almost exclusively to several astronomical events such as solar or lunar eclipses.
Plötzlich herrschte im ganzen Haus Finsternis.
Suddenly there was darkness throughout the whole house.
In an astronomical context, 'Finsternis' (eclipse) is most commonly used in the compound words 'Sonnenfinsternis' (solar eclipse) and 'Mondfinsternis' (lunar eclipse). These words are very specific and make the meaning immediately clear.
Für die Beobachtung der Sonnenfinsternis braucht man eine spezielle Brille.
You need special glasses to observe the solar eclipse.
The figurative meaning of 'Finsternis' (darkness) is often used in historical, philosophical, or literary contexts. It describes negative states such as ignorance, hopelessness, or evil.
Die Zeit des Krieges war eine Epoche der Finsternis für das ganze Land.
The time of war was an era of darkness for the whole country.
light
'das Licht' and 'die Finsternis' are clear opposites. 'Licht' means brightness that lets you see. 'Finsternis' means very deep darkness, with almost no light. In everyday language, people often simply say 'Dunkelheit' (darkness); 'Finsternis' sounds stronger and somewhat more elevated or dramatic. When a lamp turns on, light comes into a room. When there is no light, there is 'Finsternis'.

Im Wald war es so spät, dass wir in der Finsternis kaum den Weg sahen.
It was so late in the forest that we could barely see the path in the darkness.
Nach dem Stromausfall machte sie das Licht in der Küche wieder an.
After the power outage, she turned the light in the kitchen back on.