Dream is the series of images, thoughts, and feelings you experience while you're asleep. Sometimes dreams feel incredibly real: you might dream you're flying over your neighborhood, talking to people who don't exist, or reliving something that happened earlier that day. Other times dreams are bizarre and confusing, like showing up to school in your pajamas or finding a secret room in your house.

Scientists still don't fully understand why we dream, though they know dreams happen mostly during a sleep stage called REM (rapid eye movement) sleep. Some dreams are so vivid you remember them clearly in the morning, while others vanish the moment you wake up. Nightmares are frightening dreams that can wake you up feeling scared or upset.

The word also means a cherished hope or ambition. When you dream of becoming a marine biologist or a video game designer, you're imagining a future you want to work toward. These kinds of dreams give people direction and motivation. Martin Luther King Jr.'s famous “I Have a Dream” speech described his vision of racial equality in America.

People sometimes describe something wonderful as a dream come true, meaning a hope they've held has finally happened. When something goes perfectly, you might say it works like a dream.