'Rain' has meant exactly the same thing for six millennia — one of the most semantically stable words alive.
Water falling in drops condensed from vapor in the atmosphere.
From Old English 'regn' (rain, rainfall), from Proto-Germanic *regna- (rain), of uncertain but probable PIE origin, possibly *h₃reǵ- (to moisten, to wet, to flow). The Germanic forms are highly consistent: Old Saxon 'regan,' Old High German 'regan,' Old Norse 'regn,' Gothic 'rign' — all pointing to a stable Proto-Germanic root unchanged since the proto-language. Latin 'rigāre' (to wet, to water, to irrigate), from the same probable PIE