Other forms: rains, raining, rained
Rain is water falling in drops from the sky. Heavy rain, in the rain, it is raining.
It forms when droplets in cloud grow heavy enough to fall, and anything under half a millimetre across is drizzle rather than rain. English has an unusual number of words for it — drizzle, shower, downpour, deluge — which says something. Note it never rains but it pours; right as rain, meaning perfectly well; take a rain check, which is American and comes from baseball tickets.
1nounrainWater falling in drops from the sky.
“Take an umbrella, because heavy rain is expected this afternoon.”
2verbfallTo fall as rain.
“It rained all day, so we stayed inside and played games.”
Too few real examples of this word to estimate which sense is commonest.
rain is precipitation.
Old English regn is Germanic and its earlier history is uncertain, possibly from a root meaning to flow. It is one of the oldest and plainest words in the language with no clear ancestor.
Middle English
reyn
English, today
rain
From Middle English reyn. Middle English reyn, rein, from Old English reġn, from Proto-West Germanic regn, from Proto-Germanic regną, of uncertain origin. Possibly from pre-Germanic Hréǵ-no-, from Proto-Indo-European Hreǵ- (“to flow”), although the consonant reflexes don't match. 2
Etymology from Wiktionary, CC BY-SA