der Albtraum
nightmare
A very bad, frightening dream from which you often wake up.
Ich hatte letzte Nacht einen furchtbaren Albtraum von Spinnen.
I had a terrible nightmare about spiders last night.

nightmare
Loading your saved state.
nightmare
A very bad, frightening dream from which you often wake up.
Ich hatte letzte Nacht einen furchtbaren Albtraum von Spinnen.
I had a terrible nightmare about spiders last night.

The word is made up of 'Alb' and 'Traum'. An 'Alb' is, in old legends, an evil spirit or demon that sits on the chest of sleeping people at night and causes fear. So an 'Albtraum' (nightmare) is literally the 'dream caused by an Alb'.
Früher glaubten die Menschen, dass böse Geister Albträume bringen.
In the past, people believed that evil spirits brought nightmares.
When referring to a bad dream, the word is often used with certain verbs. Typical combinations are 'einen Albtraum haben' (to have a nightmare) or 'aus einem Albtraum aufwachen' (to wake up from a nightmare).
Ich bin schweißgebadet aus einem schlimmen Albtraum aufgewacht.
I woke up drenched in sweat from a terrible nightmare.
In the figurative meaning for a terrible experience, 'Albtraum' (nightmare) is often intensified with adjectives like 'absolut' (absolute), 'rein' (pure), or 'wahr' (true). A situation can also 'zum Albtraum werden' (turn into a nightmare).
Die Renovierung unseres Hauses wurde zum reinsten Albtraum.
The renovation of our house turned into a complete nightmare.
dream
'der Traum' and 'der Albtraum' can both refer to something you experience while sleeping. The difference is the mood: a 'Traum' can be neutral, pleasant, or even a wish. An 'Albtraum' is always bad, frightening, or very unpleasant. In everyday life, you can also use 'Albtraum' for a very bad situation. The words can be confused because an 'Albtraum' is a special kind of 'Traum'. If you are only talking about an experience during sleep, use 'Traum'. If it is scary or very negative, use 'Albtraum'.

Der Unfall auf der Autobahn war für alle Beteiligten ein Albtraum.
The accident on the highway was a nightmare for everyone involved.
Ihr großer Traum ist eine Reise um die Welt.
Her big dream is to travel around the world.