A link that allows you to go visit sources that link to the file you are currently looking at. This blog has a list of Backlinks at the bottom of every page. If you are on a computer with a mouse, you can hover over the backlinks to see the context. I strongly believe that Backlinks should include context.
Called a Linked Mention by Obsidian, similar to how social media notifies an account when it is Mentioned.
Less Wrong calls them Pingbacks
Other people's opinions
By far the best article I've found about backlinks is Maggie Appleton's online essay: A Short History of Bi-Directional Links. Here's a snip:
The idea of the bi-directional link goes all the way back to 1945, when Vannevar Bush dreamed up the memex machine. Vannevar outlined this hypothetical gadget in an essay in The Atlantic called As We May Think. He wanted a system capable of “associative indexing... whereby any item may be caused at will to select immediately and automatically another... (so that) numerous items have been thus joined together to form a trail.”
Zettelkasten.de says backlinks are bad:
Backlinks are bad linking on crack and steroids. Not only do they not provide any link context other than the title, but they are placed automatically. That means that your system (executed by software or habit) provides you with more choice and therefore more cognitive load. On top, you will check the backlinks if provided and that creates opportunity costs. Reviewing the backlinks is time you don’t spend with other cognitions. You will spend more time with wasting time.
I don't agree, but you might be interested in his article regardless.