A trackback is a little automatic message between websites. If you write a blog post and link to somebody else's post, a trackback tells that other site 'someone over here has linked to you', and a note about your post can then appear under theirs. It was a way for blogs to talk to each other in the days before Facebook and Twitter, roughly 2002 to 2010, and it is now mostly a dead word — the systems still exist in blog software like WordPress, but spammers abused them so badly that most sites switched them off. So if you meet 'trackback' today, it is usually in an old article, in website settings, or from someone talking about how the internet used to work. You will also see the near-twin word 'pingback', which does almost the same job by a slightly different method; ordinary people use the two words loosely, as if they were the same.

引用通知、回溯連結（部落格時代的功能，現在少用）