Universe
The universe is everything that exists: all matter, energy, space, and time, including every galaxy, star, and planet.
Definition
The universe is everything there is: every star, planet, and galaxy, all the matter and energy, and all of space and time that holds them. It is organized in layers, with planets orbiting stars, stars gathered into galaxies, and galaxies grouped into clusters, and scientists estimate it is about $13.8$ billion years old. On the largest scales it is described by general relativity and the Lambda-CDM cosmological model, in which ordinary matter makes up only about $5\%$ of the contents, with dark matter near $27\%$ and dark energy near $68\%$. The observable universe, the region whose light has had time to reach us, is about $93$ billion light-years across, and the whole universe may be far larger than that.
Example
Every star you see at night belongs to the universe, along with the Sun, the Moon, and the ground under your feet. Beyond our own Milky Way, the observable universe holds hundreds of billions of galaxies, each with billions of stars. The cosmic microwave background, the afterglow of the hot early universe, arrives from every direction at a temperature of about $2.7\ \text{K}$, and its tiny variations of about one part in $100{,}000$ map the seeds that grew into the galaxies we see today.
Key Insight
The universe is so big that distances are measured in light-years, and looking far out means looking far back in time, because light takes time to travel. A telescope aimed at a galaxy $10$ billion light-years away shows that galaxy as it was $10$ billion years ago. The universe also has no center and no known edge: the Big Bang did not happen at one spot in space, because space itself has been stretching everywhere since the beginning.