What is a unique enemy?

I'm kinda surprised this hasn't been answered on this forum yet. Certain things, like the tree passive "Arena Challenger", refer to unique enemies. I'm pretty sure the enemies with red names are unique (they have a "unique" name). What about story bosses? Map bosses? I need to know before making a decision in game. Thanks for the help.

https://www.poewiki.net/wiki/List_of_unique_monsters#:~:text=Listed%20below%20are%20only%20the%20unique%20monsters%20that,unique%2C%20but%20are%20not%20featured%20in%20this%20list.


A unique monster.
Notice its health bar has a bronze framing with dents on the top and bottom.

Thanks, LoTharios! For others who didn't follow your link, the answer is that all the types I asked about are indeed unique (those with red names like in the picture, map bosses and story bosses).