What is the noun long?

"Long" is an adjective, not a noun. It describes a quality, such as length or duration.

For example:

* Long journey.

* Long time.

To use "long" as a noun, you would need to use it in a different context. For example:

* "The long of it is, we're going to be late."

Here, "long" refers to the essence or gist of something.