Planet
A planet is a large round body that orbits a star and has cleared other objects from the neighborhood of its orbit; the solar system has eight.
Definition
A planet is a big, round object that travels around a star and shines by reflecting starlight rather than making its own; our solar system has eight, including Earth. By the definition adopted by the International Astronomical Union in 2006, a planet orbits the Sun, is massive enough for its gravity to pull it into a nearly round shape, and has cleared the neighborhood around its orbit, which is why Pluto, which shares its region with many Kuiper belt objects, was reclassified as a dwarf planet. Planets form by accretion in a protoplanetary disk: rocky planets from planetesimals inside the frost line, and giants from cores large enough to capture nebular hydrogen and helium. Thousands of exoplanets orbiting other stars have been found since 1995, mainly by the transit and radial velocity methods, revealing types such as hot Jupiters and super-Earths that do not exist here.
Example
Venus is often the brightest object in the evening or morning sky after the Moon; it looks like a star but is a planet reflecting sunlight. Jupiter, the largest planet, has a diameter about $11$ times Earth's and a mass about $318$ times Earth's, while Mercury, the smallest, is about $38\%$ of Earth's diameter. A transiting Earth-sized planet crossing a Sun-sized star dims it by $(R_{\text{Earth}}/R_{\text{Sun}})^2 \approx (1/109)^2 \approx 8.4 \times 10^{-5}$, less than one part in $10{,}000$.
Key Insight
The word planet comes from a Greek word for wanderer, because planets drift across the star patterns from night to night while the stars hold still. Planets barely twinkle: their tiny disks average out the atmosphere's shimmer, while a star is a single point that jumps around. The requirement to clear the orbital neighborhood is what separates planets from dwarf planets; Earth is about a million times more massive than everything else near its orbit combined, while Pluto is only a small fraction of the mass in its region.