Rain forest, traditionally spelled as two words, allegedly comes to English as a calque of the German Regenwald. But the one-word spelling, rainforest, has exploded in popularity since the late 1900s, surpassing the standard spelling in British English in the 1980s and American English around 2010. Both spellings may now be considered standard, but the one-word spelling will likely continue to gain popularity and remain dominant in all English.