The Battle Frontier is a location found in the Pokémon series that contains different segments each focusing on a different style of Pokémon battling. So far the Battle Frontier has appeared in Pokémon Emerald and Pokémon Platinum. Like earning badges from gyms, the Frontier Brains as the counterpart of gymleaders offer a "symbol" when defeated by a Trainer.
The Kanto Region Battle Frontier, commonly referred to as the Hoenn Region Battle Frontier because Ash Ketchum first appeared to challenge after attending the Hoenn League, is the first to appear in gaming and in the Pokémon anime. The Kanto Frontier consists of seven members training with random Pokémon, occasionally their team consists of legendary Pokémon such as Articuno. One of those is Nate, on of the first members in the Battle Frontier. In the game Pokémon Emerald however, the Kanto Region Battle Frontier is located in Hoenn.
The Sinnoh Region Battle Frontier has not yet appeared in the anime, however, it is accessible in Pokémon Platinum. The Frontier consists of five Frontier Brains, one group has two and automatically becomes a double-battle. The Trainer earns Battle Points instead of symbols, which may be earned in the anime if the Sinnoh Battle Frontier makes an appearance.
The Johto Region Battle Frontier has not appeared in any of the animes, however, you can access it in Pokémon HeartGold and SoulSilver. It is strikingly similar to the Sinnoh Region Battle Frontier except for the Frontier Access which appears in the Johto Region Battle Frontier but not in the Sinnoh Region Battle Frontier.