(covers information from several alternate timelines)
A gravity plate, also referred to as grav-plating, was used to simulate a gravity environment on starships. Gravity plating could be lined along a ship's exterior hull, or across the interior deck plates.
In 2151, when Enterprise NX-01 was preparing to engage an unknown enemy vessel, Commander Tucker and Lieutenant Reed intentionally overloaded the phase modulators to increase the yield of Enterprise's phase cannons, using the gravity plating of the affected decks to absorb the recoil. (ENT: "Silent Enemy")
In the mirror universe in 2155, Captain Jonathan Archer used the USS Defiant's gravity plating to increase the gravity under Slar, weighing him down to the ground. (ENT: "In a Mirror, Darkly, Part II")