My understanding is it merely meant sites not indexed by search engines. Your employers internal websites or the sites for your college coursework would count.
"Not indexed" is the deep web. The dark web is ones that use alternate protocols on top of the web so they can't be indexed by web search engines (includes things like Tor and Discord).
I suppose ones that require authentication (like internal employer sites) could also be dark web.
It's definitely hidden behind a login, and is often the source of many controversies including underage users, where it's easier for predators to groom and prey upon these kids. Trying to figure out a label such as "dark web" is less about the actual definition, and more about the intention behind the technology (IMO, of course).
Don't forget bit torrent. There are teenagers out there committing several world GDP worth of piracy on the daily if you believe the entertainment industry
I suppose ones that require authentication (like internal employer sites) could also be dark web.