rainforest UK /ˈɹeɪn.fɒɹ.ɪst/US /ˈɹeɪn.fɔɹ.ɪst/ noun1 •A forest in a climate with high annual rainfall and no dry season.Perhaps the most impressive expression of species diversity is the density of species found in a given area of rainforest. — 2002 formsrainforests