A gas giant (sometimes also known as a Jovian planet after the planet Jupiter) is a large planet that is not primarily composed of rock or other solid matter. Gas giants may have a rocky or metallic core—in fact, such a core is thought to be required for a gas giant to form—but the majority of its mass is in the form of gas (or gas compressed into a liquid state), mainly hydrogen and helium. Unlike rocky planets, which have a clearly defined difference between atmosphere and surface, gas giants do not have a well-defined surface; their atmospheres simply become denser toward the core. Thus, terms such as diameter, surface area, volume, surface temperature and surface density may refer only to the outermost layer visible from space.

There are four gas giants in our solar system: Jupiter, Saturn, Uranus, and Neptune. Uranus and Neptune may be considered as a separate subclass of giant planets, 'ice giants', or 'Uranian planets', as they are mostly composed of ice, rock and gas, unlike the "traditional" gas giants Jupiter or Saturn. However, they share the same qualities of the lack of the solid surface; their differences stem from the fact that their proportion of hydrogen and helium is lower, due to their greater distance from the Sun.