What is a Slug?

Hi, second newbie question here: What is a slug?
(it’s not explained in the docs)

I see there is a “slug” Pattern that matches text like a variable name. And the assets may have a slug name.
So, is it like a way to identify an asset? Is it unique in an App?

e.g.: https://prettylinks.com/2018/03/url-slugs/

So just a string with some formatting restrictions.