Is rain check a compound noun?

No, "rain check" is not a compound noun. It's a phrasal noun, meaning it's a group of words that function as a single noun.

Here's why:

* Compound nouns are formed by combining two or more words, often written together (like "raincoat") or with a hyphen (like "mother-in-law").

* Phrasal nouns are phrases that act as a single noun. They often consist of a noun and a modifier (like "rain check").

In the case of "rain check," the phrase itself refers to a specific concept (a promise of a future opportunity). The individual words "rain" and "check" don't hold individual meaning within the phrase.