Generation

Generation is a grouping used for Pokémon media released in a certain time period in real life, especially games of the core series. A generation is defined by how many Pokémon exist at the time of the media's release. Generations also make extra attempt to introduce new features and mechanics to the world of Pokémon. An example is that , though set in the same time period and region as , are not Generation I games because they were released years after the original and have many new features and differences, which makes them incompatible with the original. Since they have the same features as and were released around the same time, they are Generation III games. The lifespan of a generation typically ranges from three to five years.
==Themes==
Throughout each generation, there is usually a theme to the region introduced in a specific generation. As a result, each region and generation have several key features, such as important Pokémon and places, that place emphasis on those themes. In particular, the very last Pokémon introduced in that region and in that generation embody that generation's theme in some way. Here is the list of themes that are associated with the generations and regions:
===Generation I (Kanto): Science===
The starters of the Kanto region, Bulbasaur, Charmander, and Squirtle, are all based on animals that existed on Earth thousands of years ago (mammalian reptiles, salamanders, and turtles respectively). Eevee (the Evolution Pokémon) and its first three Evolutions were introduced here. There is a museum in Pewter City where fossil Pokémon may be revived. The game plays with the relationship between science and paranormal events during the Lavender Town affair. Porygon, the Virtual Pokémon, is introduced in this generation. Mewtwo is a genetics experiment gone wrong in a similar vein as Frankenstein. The generation's final two Pokémon, Mewtwo and Mew, embody the two main aspects of science that are covered in that region: technology and genetics respectively.
===Generation II (Johto): Cultural Traditions===
As Gold and Silver are direct sequels to Red and Blue, returning players may follow the story of Team Rocket, who are still desperately trying to band back together since the defeat and disappearance of Giovanni. Unlike in the real-world mafia, heavily dictated by understood rules and carefully coordinated, the succession plan for Rocket wasn't nearly as concrete, since Giovanni's disappearance was so unexpected. Still, they are tied to the rules he set in place, and try in vain to contact him or his planned successor, Silver. The rest of the Region has a much better relationship with tradition, with many shrines, festivals, and historical events to discover.
===Generation III (Hoenn): The Environment===
The teams of the Hoenn Region believe that the environment is in danger and that the awakening of their chosen Legendary Pokémon (Groudon for Team Magma in Ruby, Kyogre for Team Aqua in Sapphire) will save it (by either droughting the sea and expanding the land, or flooding the earth and expanding the ocean respectively). A focus is also placed on the sky as a balancing force between land and sea in the form of Rayquaza, a Pokémon that can stop feuds between Kyogre and Groudon. The counterpart to Rayquaza is Deoxys, a Pokémon from space that came to the Pokémon earth born on a meteorite which was eaten by Rayquaza. Deoxys represents the environment on other worlds as well as the adaptations of living on other planets. The Legendary Giants represent the ages of humanity and the change of the planet's environment with them, with Regice symbolizing the Ice Age, Regirock the Stone Age, and Registeel the Iron Age.
===Generation IV (Sinnoh): Mankind, Legends, and the Universe===
Team Galactic possesses the most extreme goal yet: utter destruction of the universe and replacement with a copy "without spirit," or in other words dictated strictly by rationale. The Lake Guardian trio were responsible for giving humanity the traits which Cyrus considers part of a person's "spirit," with Uxie giving them knowledge, Mesprit bestowing emotions, and Azelf giving willpower. By oppressing these Pokémon, Team Galactic hope to enslave Palkia (who wished space into existence) and/or Dialga (who did the same for time) to help them achieve their end goal. The relationship the Creation Trio have is reminiscent of that shared by the real-world mathematical model of physics, spacetime, wherein all three dimensions of space are fused with time as a fourth dimension. Although not directly related to the study of spacetime, matter (and thus antimatter) bends reality around itself in a 4-dimensional plane, creating gravity, the phenomenon by which planets are gradually formed. The people of Celestic Town keep the legends of the Sinnoh Region, which turn out to be facts, alive in the zeitgeist, with Cynthia studying the mythos in her free time and often discussing it with the player. Sinnoh also contains the only dedicated church in all of the world seen so far: Hearthome's Foreign Building. The Sinnoh Region was the origin point for the rest of the world, so it is little surprise that it may contain some of the most spiritual or superstitious people.
===Generation V (Unova): Conflict===
Reshiram and Zekrom represent two sides of a conflict that happened many years before the events of Black and White. Kyurem represents neutrality, as well as the remainder of the world when there are two sides to fight on. Furthermore, the fusion of Kyurem with either Zekrom or Reshiram shows these neutral forces joining on one side of a battle to gain a somewhat unfair advantage. The original dragon from whence the Tao Trio came symbolized how easily sentient nature can divide and devolve from peace into conflict. The names of the games themselves, along with the colors of their respective Legendary Pokémon, show that the two are mutually exclusive and opposed forces. Victini represents the elusiveness of victory, something that only a mythical force joining one side of a conflict can bring, if those forces are perfectly matched (it has also been suggested that Victini was loosely based on atomic warfare, Japan being the heaviest victim of this). The Swords of Justice are veterans of a mighty siege during this war, in which each did notable heroic deeds such as protecting other Pokémon. Genesect symbolizes genetic modification of creatures for use in battle. 
===Generation VI (Kalos): Life, Death and the Ecosystem===
===Generation VII (Alola): Day and Night===
===Generation VIII (Galar): England and Its Culture===
===Generation IX (Paldea): Past vs. the Future===
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