A gas giant is a generic astronomical term invented by the science fiction writer James Blish to describe any large planet that is not composed mostly of rock or other solid matter. Gas giants may still have a solid core - in fact, it is expected that such a core is probably 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). Unlike rocky planets, gas giants do not have a well-defined surface.
From top: Neptune, Uranus, Saturn, and Jupiter ()
The term 'gas giant' is actually somewhat of a misnomer. For example, Jupiter has a thick atmosphere composed of mostly hydrogen gas and helium, with trace amounts of other chemicals such as ammonia. However, the majority of the planet's mass is liquid hydrogen, possibly with a rocky or nickel-iron core. The composition of the other gas giants is similar, though Uranus and Neptune have more water, ammonia, and methane. The lower layers of liquid hydrogen inside gas giants is often so highly compressed that it becomes metallic in nature; metallic hydrogen is stable only under such enormous pressures.
Many of the extrasolar planets which have been discovered have masses of several times Jupiter's mass, and on the basis of this it has been suggested that these may be gas giants. However, it is important to note that the detection techniques that have been used to identify extrasolar planets so far (detecting doppler shift in the star's spectrum due to the wobble induced by the planet's orbit) are much more adept at detecting giant planets than smaller ones and therefore this sample may be biased. In addition, with a few exceptions, the actual composition and structure of extrasolar planets have not been observed and many of the extrasolar planets are much closer to their parent stars and hence much hotter than gas giants in the solar system, making it possible that some of those planets are a type not observed in the solar system.
The upper mass limit of a gas giant planet is approximately 70 times that of Jupiter (around 0.08 times the mass of the Sun). Above this point, the intense heat and pressure at the planet's core begins to induce nuclear fusion and the planet ignites to become a red dwarf. Interestingly there appears to be a mass gap between the heaviest gas giant planets detected (about 10 times the mass of Jupiter) and the lightest red dwarfs. This has led to suggestions that the formation process for planets and binary stars may be fundamentally different.