Is the word behind a noun?

The word "behind" is a preposition, not a noun. It's used to show the position of something relative to another thing.

Here are some examples:

* The cat is behind the sofa. (Here, "behind" shows the cat's position in relation to the sofa.)

* I left my keys behind the door. (Here, "behind" shows the location of the keys relative to the door.)

So, "behind" itself is not a noun, but it can be used in phrases that contain nouns.