NU

“NU (NeverUsed) is Smogon’s fourth usage-based tier. All Pokémon that are not OU, UU, or RU by usage and are not banned to Ubers, UUBL, RUBL, or NUBL can be used in NU, including Pokémon from PU. While any of the aforementioned Pokémon can be used in NU, only those Pokémon that make up at least ~4.52% of total weighted usage in the tier are designated NU. A Pokémon is truly NU if a typical competitive player is more than 50% likely to encounter that Pokémon at least once in a given day of playing (15 battles).” -Smogon.com