Zero gravity or null gravity is the natural freefall environment of space, where there's the illusion of weightlessness despite the surrounding objects still exert gravitational force.
By 2151, artificial gravity had been invented to counter zero gravity. However, every ship still had an area where zero gravity was maintained. Travis Mayweather called the area the "sweet spot" during his youth aboard the ECS Horizon. (ENT: "Broken Bow")
A location aboard the ECS Fortunate was intentionally kept in a zero, or low, gravity environment. There, members of the crew could throw a football through the low gravity area while standing in an artificial gravity zone. (ENT: "Fortunate Son")
Although artificial gravity was usually maintained in most sections of a starship, a vessel's environment can revert to its natural zero gravity state when its artificial gravity systems fail. (ENT: "Broken Bow", "Unexpected", "Horizon"; Star Trek VI: The Undiscovered Country; VOY: "Prey")