What is the noun of contain?

The noun form of "contain" can be either "content" or "container", depending on the context.

Content refers to the things that are held within something. For example:

* "The content of the box is a surprise."

Container refers to the thing that holds the content. For example:

* "The container is made of metal."

So, the noun form of "contain" depends on what you are focusing on: the thing being held or the thing holding it.