Jungle is the name given to the second main expansion of the Pokémon TCG, released a few months after Base Set. In Japan, it was released as Pokémon Jungle. It's based on Pokémon Red, Blue, and Green, featuring Pokémon from Generation I.
The English expansion was released on June 16, 1999, over two years after the Japanese expansion was released, on March 5, 1997. It was originally planned to be released in the United States around August, 1999 (and rumored as late as October). However, since Topps, a competitor, was releasing their Pokémon trading cards earlier in the summer, Wizards decided to push its English 1st edition Jungle release far ahead of schedule, to June 16. Within two weeks, 1st Edition Jungle was sold out, so Unlimited Jungle was released shortly thereafter.