Is the word rain a noun?

Yes, "rain" is a noun. It refers to the water that falls from the clouds as precipitation.

Here are some examples:

* "The rain was coming down in sheets." (The word "rain" is the subject of the sentence.)

* "We got caught in the rain." (The word "rain" is the object of the preposition "in".)

* "The rain was so heavy that the river overflowed." (The word "rain" is the subject of the sentence.)