A backlink or bi-directional link is a link from one web page or piece of content back to another page or piece of content which is either related, similar, or a copy.
Bi-directional links can show social awareness by indicating knowledge about other pages or network nodes that point to it by making these links explicitly visible to people. This can enable two-way conversation between spaces on the web.
Depending on the context, these links can have subtly different connotations, different use cases, and dramatically different implementations. Some of these use cases can be found below.
Webmention is a W3C recommendation that allows one web page to send a notification to another web page to indicate it has been linked to.
In practice, many sites that send and receive these notifications actually parse the pages of the incoming notifications to show additional context from the sending page including a backlink to the originating site. As a result Webmentions become an optional but very rich way to create bi-directional links between related pages as well as encouraging the sharing of some additional context between these pages.
It is common practice for posts on websites to be cross-posted or syndicated to social platforms. Articles are often syndicated with a title and a backlink to allow readers to visit the original and see the entire context of the post.
In many cases, backlinks to other post types are included in syndication to aid in creating a one-to-one relationship between an original post and a syndicated copy to aid in original-post-discovery for performing backfeed.
The rel-me microformat is frequently used on links and backlinks to indicate that two pages represent the same identity. This can underpin the ability to perform distributed verification as well as to enable RelMeAuth.
Digital gardens, wikis, commonplace books, zettelkasten, etc. are often built with the ability to (automatically) create backlinks from one page to another to create relationships between knowledge within them. Most often, these backlinks only exist within the platform or site being used and don't show backlinks from external sources. There are, however, some of these platforms for which people are implementing Webmentions to allow for backlinks to external content.
- MediaWiki has provides a default link on each page named "What links here" which you can click and see a list of all the pages that link to the current page. With some additional work, one could transclude this page to provide backlinks on the original page. MediaWiki also has its own custom search page for this functionality: https://indieweb.org/Special:WhatLinksHere
- Roam Research, a silo-based digital notebook service, popularized the idea of bi-directional links in the digital notebook space in 2019. These backlinks only work automatically within one's notebook and do not work with external services.
- TiddlyWiki, a digital notebook, has a few flavors or plugins inspired by Roam Research to allow the creation of backlinks
- Notion: In 2020, Notion, ostensibly to compete with Roam Research, added backlink functionality to their product.
In the Search Engine Optimization context a backlink is a link created when one website links to another. Here these backlinks are sometimes also referred to as "inbound links" or "incoming links." Backlinks are valuable for SEO because they represent a signal from independent websites that other peer sites contain valuable or worthwhile content. These backlinks can therefore impact one's search ranking and visibility on a Search Engine Results Page (SERP).