Terrain is a field effect that affects grounded Pokémon for a limited number of turns. Electric Terrain can support Electric-type attacks and prevent sleep in many games, Grassy Terrain can restore HP and modify certain Ground-type interactions, Misty Terrain can reduce some Dragon damage and prevent certain status conditions, and Psychic Terrain can block priority attacks against grounded targets. Exact effects vary by generation. Flying types and Pokémon with certain effects may not be grounded, so they may not receive terrain benefits. Terrain teams should be built around the effect rather than using it randomly.
