Terrain is a set of field effects that benefit grounded Pokemon for 5 turns. Each of the four types of terrain has a move and Ability that start the effect. The moves are Electric Terrain, Grassy Terrain, Misty Terrain, and Psychic Terrain, and the corresponding Abilities are Electric Surge, Grassy Surge, Misty Surge, and Psychic Surge. When a different terrain effect comes into play, it replaces the old one. If the Pokemon that started the effect is holding a Terrain Extender, it lasts 8 turns instead.
Some moves have altered behavior when used while any terrain is active, including Camouflage, Nature Power, and Secret Power. Certain Abilities are also activated while a specific terrain is in play, which include Grass Pelt during Grassy Terrain, and Surge Surfer during Electric Terrain. There are four items that activate as soon as the holder is on the field at the same time as the corresponding terrain effect, which are Electric Seed, Grassy Seed, Misty Seed, and Psychic Seed. The effects of all of these moves, Abilities, and items happen even if the Pokemon is not grounded.
Pokemon that are in the invulnerable turn of a two-turn move are not considered grounded by terrain effects.