metamorphosis and transform into a similar but stronger species of Pokémon, a process called evolution; this process occurs spontaneously under differing circumstances, and is itself a central theme of the series. Some species of Pokémon may undergo a maximum of two evolutionary transformations, while others may undergo only one, and others may not evolve at all. For example, the Pokémon Pichu may evolve into Pikachu, which in turn may evolve into Raichu, following which no further evolutions may occur. Pokémon X and Y introduced the concept of "Mega Evolution," by which certain fully evolved Pokémon may temporarily undergo an additional evolution into a stronger form for the purpose of battling; this evolution is considered a special case, and unlike other evolutionary stages, is reversible. In the main series, each game's single-player mode requires the Trainer to raise a team of Pokémon to defeat many non-player character (NPC) Trainers and their Pokémon. Each game lays out a somewhat linear path through a specific region of the Pokémon world for the Trainer to journey through, completing events and battling opponents along the way (including foiling the plans of an evil team of Pokémon Trainers who serve as antagonists to the player). Excluding Pokémon Sun and Moon and Pokémon Ultra Sun and Ultra Moon, the games feature eight powerful Trainers, referred to as Gym Leaders, that the Trainer must defeat in order to progress. As a reward, the Trainer receives a Gym Badge, and once all eight badges are collected, the Trainer is eligible to challenge the region's Pokémon League, where four talented trainers (referred to collectively as the "Elite Four") challenge the Trainer to four Pokémon battles in succession. If the trainer can overcome this gauntlet, they must challenge the Regional Champion, the master Trainer who had previously defeated the Elite Four. Any Trainer who wins this last battle becomes the new champion. Pokémon universe Pokémon is set in the fictional Pokémon universe. There are numerous regions that have appeared in the various media of the Pokémon franchise. There are 9 main series regions set in the main series games: Kanto, Johto, Hoenn, Sinnoh/Hisui, Unova, Kalos, Alola, Galar, and Paldea. Each of the nine generations of the main series releases focuses on a new region. Every region consists of several cities and towns that the player must explore in order to overcome many waiting challenges, such as Gyms, Contests and villainous teams.