Generate a topic word for the given sentence. A topic word is usually the name of a person, place, animal, concept or object the sentence is focusing on.

[Q]: nightmare disorder, also known as "dream anxiety disorder", is a sleep disorder characterized by frequent nightmares.
[A]: nightmare disorder


[Q]: rsgc1 is a massive young open star cluster in the milky way galaxy.
[A]: rsgc1


[Q]: the economy of azerbaijan is based on industry, agriculture, and on services including tourism.
[A]:
economy of azerbaijan