What is Atom in WordPress?

Sitemap can be in any format XML, RSS and Atom feed The main difference between these is that the XML Sitemap submits the details of the URL of all the pages and posts of your website to the Google webmaster.

Whereas the RSS / Atom feed describes all the changes happening in your website and updates it in the webmaster.

XML sites are very large because they keep all the data of your website in url format whereas RSS/Atom feeds are small files as they only update the recent changes.

In WordPress, an Atom feed is a type of web feed that allows users to subscribe to a website's content and receive updates when new content is published. Atom feeds are one of several types of web feeds that are supported by WordPress, the other being RSS (Really Simple Syndication) feeds.

An Atom feed contains a list of recent posts from a website, along with metadata such as the post's title, author, and date of publication. Users can subscribe to an Atom feed using an RSS reader or feed aggregator, which will automatically check for and display new content.

In WordPress, the default Atom feed is typically accessible at the website's address followed by /feed/atom/. For example, if your website is example.com, the Atom feed would be available at example.com/feed/atom/.

Atom is a format to publish content in XML to be used by feed readers and news aggregators.
This is known as the Atom Syndication Format.