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.

Example Input: nightmare disorder, also known as "dream anxiety disorder", is a sleep disorder characterized by frequent nightmares.
Example Output: nightmare disorder

Example Input: metis is the goddess of wisdom in greek mythology.
Example Output: metis

Example Input: xtra airways is a charter airline from the united states.
Example Output:
xtra airways