Base Set is the name commonly used for the first Pokémon Trading Card Game set.
The set was released in Japan on October 20, 1996, where it was known as the 1st Starter & Expansion Pack. In English-speaking markets, it launched as Base Set on January 9, 1999. The difference in naming can make the Japanese and English releases seem like separate sets, but they represent the original core release.
Base Set contains 102 cards and features Pokémon from the first generation. Its famous cards include Charizard, Blastoise and Venusaur, each used as artwork for an English booster-pack design. The Japanese release was published by Media Factory, while Wizards of the Coast produced the early English cards.
Jungle was the second English expansion, followed by Fossil. Neo Genesis came much later and introduced cards based on the second generation, so it is not part of the original Base Set era.