A Basic Pokémon is a form of Pokémon which does not evolve from any other Pokémon. A Basic Pokémon can be placed directly into play, either as the Active Pokémon at the start of the game or as a Benched Pokémon any time during play. Some Basic Pokémon, such as Mewtwo, do not evolve;
some, such as Bulbasaur, do; and some, such as Pikachu can evolve from Baby Pokémon, either classified as a Baby Pokémon or with the Baby Evolution Poké-Power. In these cases, Basic Pokémon that are evolved from Baby Pokémon or other Basic Pokémon are considered to be Evolved Pokémon.
From Bulbapedia about Pokemon Cards: