The word Porygon is a play-on of the word polygon. Polygons are plane figures with three or more sides and typically four or five, and so because of this this is where Porygon gets its name.
Porygon started in Generation I and II with no evolution. However, in the third generation, Porygon could evolve into Porygon2 whilst holding the Up-Grade during trade. Then, in the fourth generation, Porygon could evolve into Porygon-Z whilst holding the Dubios Disc in trade.