rain·for·est or rain forest (r?n?fôr??st, f?r?-)n. A dense evergreen forest with a minimum annual rainfall of approximately 180 centimeters (71 inches). Rainforests are found chiefly in the tropics but also occur in temperate regions, where the rainfall amount is somewhat lower.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.rainforest (?re?n?f?r?st) n (Forestry) dense forest found in tropical areas of heavy rainfall. The trees are broad-leaved and evergreen, and the vegetation tends to grow in three layers (undergrowth, intermediate trees and shrubs, and very tall trees, which form a canopy). Also called: selva Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014rainforestAn area of dense tropical evergreen forest with a very heavy annual rainfall.Dictionary of Unfamiliar Words by Diagram Group Copyright © 2008 by Diagram Visual Information Limited