Really Simple Discovery (RSD) is an XML format and a publishing convention for making services exposed by a blog or other software discoverable by client software.
It is a way to reduce the information required to set up editing/blogging software to three well known elements: username, password, and homepage URL. Any other critical settings should either be defined in the RSD file related to the website, or discoverable using the information provided.
RSD was authored by Daniel Berlinger in the Really Simple Discoverability 1.0 specification.
To make use of RSD, the owner of a site places a link tag in the head section of the homepage which indicates the location of the RSD file. An example of what MediaWiki uses is:
If this tag is missing or the file is not found, clients should look in the default location, which is a file named rsd.xml in the webroot.[1] For example, at <nowiki>https://example.net/rsd.xml</nowiki>.
Here is a sample RSD file, from "Really Simple Discoverability 1.0":
MediaWiki example: