Planet
In short
A large, rounded astronomical body that orbits a star, stellar remnant, or brown dwarf. It must be large enough for its own gravity to make it spherical and have cleared its orbital path of other debris.
In plain words
A planet is a massive object in space that circles a sun. It is round like a ball and is not a star itself. It stays in its own lane in space without many other big objects in its way.
A simple example
Earth is the planet we live on, which orbits the Sun. Jupiter is another example, known for being a massive gas giant.
Why it matters
Understanding planets helps scientists learn how solar systems form and where life might exist. It also allows us to track how gravity shapes the universe.
Easy to mix up
Do not confuse a planet with a moon, which orbits a planet instead of a star.