An astronaut is a person who travels into space.
If someone says they're going to become an astronaut one day, it means they want to join a special group of people who get to explore the universe and work on spaceships. You might see astronauts wearing cool suits and helmets when they go out in public after a long trip. People often talk about astronauts when discussing space missions or when there's a new discovery that requires someone to travel to outer space.
A member of the crew of a spaceship or other spacecraft that travels beyond Earth's atmosphere, or someone trained to serve that purpose.
