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 related Basic Pokémon in play (including Fossils or Pokémon that have evolved from Baby Pokémon) at any time before the player's attack. Stage 1 Pokémon are generally stronger than the Basic Pokémon before them. Stage 1 Pokémon cannot evolve on the turn the Basic Pokémon was played, or on the player's first turn. When placed onto the Basic Pokémon card, all Energy Cards, damage counters and other attached cards remain, but any Special Conditions or effects on the Pokémon are removed. Stage 1 Pokémon may only use their Stage 1 attacks unless their card specifically states otherwise. Stage 1 Pokémon cards will generally state 'Stage 1' and have a picture of the Basic Pokémon from which they evolve in the upper left-hand corner or the left center of the card, along with the text 'Evolves from <Basic Pokémon>'. Stage 1 Pokémon can be removed from the Basic Pokémon by use of Devolution.
Stage 1 Pokémon are not be confused with Basic Pokémon that evolve from Baby Pokémon, even though Baby Pokémon will still say 'Basic' on the card. For example, Pikachu is not a Stage 1 Pokémon, even though it evolves from Pichu. Pichu is a Baby Pokémon. In this case, Raichu would be the Stage 1 Pokémon because it evolves from Pikachu, a Basic Pokémon.
Some Pokémon-ex cards are Stage 1 Pokémon if that Pokémon's non-ex card is normally a Stage 1 card. For example, Wailord-ex is a Stage 1 card because Wailord is a Stage 1 card.
Outside of the TCG, Stage 1 Pokémon are commonly referred to as First-evolution Pokémon.
Pokémon such as Ivysaur, Raichu, and Cloyster are Stage 1 Pokémon. They evolve from Basic Pokémon such as Bulbasaur, Pikachu, and Shellder.