From Bulbapedia, the community-driven Pokémon encyclopedia.

A Basic Pokémon is a completely unevolved Pokémon from the Pokémon Trading Card Game.

A Basic Pokémon can be placed in play (Active or Bench) without any other Pokémon required to be there, however, some Basic Pokémon can be evolved from Baby Pokémon.

Pokémon such as Bulbasaur, Pikachu and even Mewtwo are Basic Pokémon.