Gravity is a fundamental force or interaction of attraction between bodies of mass. One of the first formal mathematical formulations was by Isaac Newton.
It is summarised with the equation:
where F is a force in newtons, G is the gravitational constant of the universe, m1 and m2 are masses in kilograms and r is the distance in metres between the centres of the two masses.
It has been refined by Einstein's Theory of General Relativity to take into account more extreme gravitational effects. For example how light and mass affect each other near massive objects like the sun.