Of the four fundamental forces known to science (the others being electromagnetism and the strong and weak nuclear forces), gravity is the weakest, but also the most inescapable. Any object with mass generates gravity as it warps spacetime. Gravity pulls objects together; given sufficient time, small dust particles in the vacuum of space will coalesce and accrete, eventually forming massive bodies like stars and planets. Sir Isaac Newton laid down the classical theory of gravity in his Principia Mathematica, describing it as a universal force regulated by an inverse-square law: F=G(m1m2)/r^2, where F is the force of gravitational attraction, G is the universal gravitational constant, m1 and m2 are the masses of the two bodies being considered, and r is the distance between them. Albert Einstein explained gravity as an acceleration - without reference to outside phenomena, it is impossible to distinguish a stationary frame of reference from and accelerating one, and a person on a constantly accelerating spaceship or a rotating space station would feel a constant gravity.
A lower-case letter g is used to denote the acceleration due to gravity on Earth - roughly 9.8 meters per second per second, or 32 feet per second per second.
In gaming, gravity is the force that constantly pulls objects and the player "down". In games, gravity's accelerating force is often denoted in either pixels per tick or game units per tick, a tick being a single cycle of the game engine's loop.
In early games, gravity was only of consequence to the player character, as only they were capable of moving freely. Recently, however, gravity has gained "first class citizen" status, as advanced physics simulation in games (such as the havok and novodex engines) enabled all objects to move and interact with the player.
In 2D games, gravity almost universally points towards the bottom of the screen, unless it is flipped by a gameplay gimmick. In 3D games, gravity almost universally points "down" in the 3D world, and the camera pivots around an axis parallel to it. Gravity's pull, like all other simulated physical forces, is often limited to a maximum value by the game engine, in order to avoid having objects falling from high places going faster than the computer can account for. Gravity is also usually an environmental hazard, as falling from a high place - and thus reaching high speed when impacting the ground - will hurt the player.
The effects of gravity are often not noticed by players as it is taken for granted, except in cases where gravity suddenly shifts or becomes absent altogether - games such as Prey, which featured fully multi-directional gravity, the Unreal Tournament series which featured arenas with reduced gravity, and Dead Space, which featured areas with no gravity at all, forcing the player to constantly reorient themselves as they stick to walls with their magnetic boots.