From Bulbapedia, the community-driven Pokémon encyclopedia.
A Stage 1 Pokémon is a type of evolution card in the Pokémon Trading Card Game. Stage 1 Pokémon evolve from Basic Pokémon.
A Stage 1 Pokémon can be placed on top of any basic Pokémon in play (including Fossils or Pokémon that have evolved from Baby Pokémon) at the beginning of the player's turn. When placed onto the basic Pokémon card, all Energy Cards and damage counters remain, but any Special Conditions are removed. Stage 1 Pokémon may only use their Stage 1 attacks, unless their card specifically states otherwise.
Pokémon such as Ivysaur, Raichu, and Omanyte are Stage 1 Pokémon.