1. A joke religion created by hardcore fans of the Pokémon franchise. Some fans claim to "follow" this religion to identify themselves with other "believers".
2. The moniker used by less fanatic players of the Pokémon franchise to refer to the in-game mythology, sparse as it is, that references the "Original One" aka Arceus.
1. "Hey Mike, are you a follower of Arceusism?"
"Definently, Arceus forever!"
"Amen to that,"
2. In the Canalave library in the Diamond, Pearl, and Platinum versions, there is a book on the third floor that tells of how the Original One emerges from an egg and creates the beings of time and space as well as the beings that gave life knowledge, emotion, and willpower. He then goes to eternal sleep.
In Platinum there is an event Arceus that allows the player to hear more of this legend as he/she discovers Plates, which Arceus uses to change type with its ability.
Someone who views Arceus as the god of the world. Generally Nintendo Fanboys.
Guy 1 - Have you seen this Arceusism thing?
Guy 2 - Yeah, I am an Arceusist.
Guy 1 - WTF man its a load of bull!
Guy 2 - Don't....
Arceus used Judgement. It was super-effective.