Outline Processor Markup Language (.opml)

OPML (Outline Processor Markup Language) is an XML-based format designed for the exchange of outline-structured information. Originating from UserLand Software, it was initially developed for the outliner tool within Radio UserLand. Today, OPML's primary use is for sharing lists of web feeds, such as RSS, between content aggregators. An OPML file comprises a root element with a version attribute, a head element containing metadata, and a body element that holds the outline's content. Each item in the outline is denoted by an outline element, capable of housing additional data through various attributes.

OPML files are beneficial for distributing structured data across diverse platforms and applications. They facilitate the management and migration of web feeds, enabling straightforward import and export of RSS feed subscriptions. The hierarchical structure of OPML is ideal for organizing information into tiered lists, which proves useful in a range of activities, from bookmark organization to intricate project planning. Due to its open and adaptable nature, OPML has become a key standard for compatibility among outlining and feed reading software, with many supporting OPML file import and export functionalities.