From Bulbapedia, the community-driven Pokémon encyclopedia.

A Stage 2 Pokémon is a type of evolution card in the Pokémon Trading Card Game. Stage 2 Pokémon evolve from Stage 1 Pokémon.

A Stage 2 Pokémon can be placed in play if the corresponding Stage 1 Pokémon is Active or Benched. Certain effects can circumvent this, such as the Pokémon Breeder Trainer card, that allows a Basic Pokémon to evolve into a Stage 2 Pokémon

Pokémon such as Venusaur and Omastar are Stage 2 Pokémon.