Jungle was the first English Pokémon Trading Card Game expansion set.
The Pokémon TCG launched in English with Base Set in January 1999. Jungle followed later in 1999 as the first expansion, adding new cards beyond the initial release and expanding the collectible card pool.
Jungle is especially associated with the first expansion-era cards for Pokémon such as Scyther, Pinsir, and the distinctive Wigglytuff. Its booster packs and artwork helped establish the recurring pattern of themed expansions.
Base Set is commonly mistaken for the answer because it was the first English Pokémon TCG product, but it was the initial set rather than an expansion. Fossil was the next major English expansion after Jungle.