A rainforest was a forest characterized by high rainfall.
On the planet Earth, the country of Brazil was known for its diverse wildlife found in the Amazon rainforest. (ENT: "Fight or Flight")
The planet Lactra VII featured several different ecologies co-existing significantly close to each other, such as a desert and a rainforest separated by a river. Maravel dragons were kept in the rainforest environment in the zoo located on the planet. (TAS: "The Eye of the Beholder")
In 2270, Ari bn Bem, a Pandronian, eluded the landing party of the USS Enterprise by splitting into several body parts amidst the rainforest on Delta Theta III. (TAS: "Bem")
On the planet Corvan II, the rainforests home to animals known as gilvos were threatened by industrial pollutants. By 2368, only 14 specimens remained. (TNG: "New Ground")