a forest with heavy annual rainfall
"The lush vegetation in the rainforest thrives due to its abundant annual rainfall."
A forest in a climate with high annual rainfall and no dry season.
"The lush vegetation of the Amazon rainforest thrives because it receives heavy rains every single day without any dry period."
In plain English: A rainforest is a very hot and wet forest with huge amounts of rainfall that supports an enormous variety of plant and animal life.
"We traveled deep into the rainforest to see exotic birds and lush green plants."
Usage: The term specifically refers to tropical or subtropical forests, so avoid using it interchangeably with temperate rainforests unless the regional distinction is relevant. It is often confused with "jungle," but while all jungles are dense vegetation, not all rainforests qualify as true jungles due to differences in tree density and understory growth.
The word "rainforest" is an English translation of the German term Regenwald, combining "rain" and "forest." It was first used to describe tropical regions with heavy rainfall and dense vegetation.