Condensed water falling from a cloud.
"The sudden rain soaked my umbrella before I could even reach the car."
In plain English: Rain is water falling from clouds as drops when it gets wet outside.
"The rain started just as we left the house."
Usage: Use "rain" as a noun to refer specifically to the liquid precipitation falling from clouds, such as in "heavy rain fell all night." Do not use it to describe the weather event itself unless you are referring to the actual moisture descending from the sky.
To have rain fall from the sky.
"Shakespeare occasionally spelled the verb to rule as rain in his early drafts before standardizing the spelling to reign."
Obsolete form of reign.
In plain English: To rain is for water to fall from clouds as droplets.
"We should stay inside because it is raining heavily outside."
Usage: Use "rain" as a verb to describe something falling or dropping heavily in large quantities, such as "money rained down from the sky." Do not use it to mean "to rule," as that usage is an obsolete form of "reign."
The word rain comes from Old English reġn and is likely related to a pre-Germanic root meaning "to flow," though its exact origin remains uncertain. It has been used in English since before the Middle English period with this core meaning of falling water.