A Basic Pokémon is a Pokémon that has not yet evolved and is not a Pre-evolved Pokemon. They tend to have low stats, usually with an average of 2-3 stats rising one unit, and occasionally, one stat (speed usually) rises by 3. A Legendary Pokemon is not a basic Pokémon- as they are not as weak as a normal Pokemon. Pokémon that don't evolve (other than Legendary Pokemon) are still basic Pokémon. Magikarp is probably the most basic Pokémon around and the hardest Pokémon to evolve in the opinion of most. In the trading card game, they are also Pokémon that have not yet evolved and is not a Pre-evolved Pokemon. This has been part of the tcg ever since the base set. Many examples of basic Pokémon are:

1.It Will Evolves Into Stage 1 to Stage 2

2.It is Not a Baby Pokémon

3.In the anime, Pokémon like those above ARE NOT Basic Pokemon. They are JUST Pokemon.