Pokedex
In the Pokemon universe, a Pokedex is a pocket-sized computer containing a comprehensive informational database regarding all known Pokemon species. Pointing the device at a Pokemon causes a computerized voice to read aloud an entry about that Pokemon variety. Over the five game generations since the original release of "Pokemon Blue Version" and "Pokemon Red Version," the device has seen many changes including a gradual shift from button inputs to a full touch-screen display. In-game, the Pokedex serves more as a cataloging system for players. It keeps track of what Pokemon the player has seen and captured, as well as providing a brief tidbit of trivia or information regarding each Pokemon.
Regional Pokedex
A regional Pokedex is a Pokedex specifically set up to catalog Pokemon native to a specific region. This concept was introduced in the third generation games "Pokemon Ruby Version" and "Pokemon Sapphire Version" and has persisted throughout. Pokemon are cataloged outside of their "correct" national order and instead are listed in order of appearance in the region, starting with the starter Pokemon of that region and moving onward. The regional Pokedexes have a "national" mode, which is generally unlocked throughout the course of gameplay. The national Pokedex includes all Pokemon included at any time, and listed in their correct numerical order instead of their regional order.
Unova/Isshu
The Unova region was introduced in the fifth generation Pokemon games, "Pokemon Black Version" and "Pokemon White Version." Named "Isshu" in the original Japanese version, this region of the Pokemon world is considered to be quite far away from the other regions of previous games. According to game director and composer Junichi Masuda, the Unova region drew inspiration from metropolitan New York City. Unova is an extremely diverse area, with many different types of people, Pokemon and environments.
Isshu Pokedex
The Isshu or Unova Pokedex is the regional Pokedex for the Unova/Isshu region. It contains precisely 155 Pokemon, starting with the special-event legendary Pokemon Victini, whose regional number is 000 and national number is 494. The Unova/Isshu Pokedex ends with another special legendary Pokemon, Genesect. At the time of publication Genesect has not been officially released, and is regional number 155 and national number 649. The Unova/Isshu Pokedex is a rare exception to Pokedex conventions with the placement of Victini at the beginning, but it otherwise follows standard convention afterward with number 001 being the Unova Grass-type starter Pokemon, Snivy.