g - Gravitational Constant

The pull of gravity around the Earth.
Credit: Wikimedia user Sjlegg

"g" is the measurement of gravity on the surface of the Earth.

It is an acceleration (how speed changes over time), and on Earth this means that when something falls, its speed increases by g, where here on Earth g=9.8 m/s2 (metres per second per second).

It is usually used as the unit of gravity for the surfaces of the other planets.

So, on a planet with a surface gravity of less than 1 g you would feel lighter and on a planet with a surface gravity of more than 1 g you would feel heavier.