Battle Frontiers (バトルフロンティア Battle Frontier) are areas in the Pokémon games, that are somewhat like a Theme Park dedicated to Pokémon battles. Every Battle Frontier has a set of unique Facilities, where players battle through a string of Trainers, and eventually fight the Facility overseer, a Frontier Brain. Currently there are two Battle Frontiers, one in Hoenn, and one in Sinnoh.
For information on one of the individual Battle Frontier parks see these articles: