means A dense, tall forest in a region of consistently heavy rainfall, teeming with an extraordinary diversity of plants and animals — found in both tropical and cooler temperate zones.

from A transparent compound of "rain" and "forest," but a surprisingly modern one: the word is a 20th-century calque (a word-for-word translation) of the German "Regenwald," coined by botanists describing the wet tropics. "Rain" traces back through Old English "regn" to a Germanic root shared with German "Regen"; "forest" arrives via Old French from Medieval Latin "forestis," meaning woodland kept outside the cultivated bounds — possibly from Latin "foris," "outside." So the name itself is young, even if the thing it describes is ancient.