Outline Processor Markup Language (OPML) is an XML file format used for creating outlines. As a result of the extensibility of the XML base, OPML has been used to organize many kinds of data as it may be customized for each application. The format is simple, self-documenting, extensible and human-readable. OPML allows a user to classify where content should be placed within a hierarchy as it is being created. This feature makes the format extremely useful for the creation of "living documents," where relationships and content are continuously updated.
OPML was created as a file format for an outliner application. An outline, in the context of information technology, is a hierarchy that explains how textual elements relate to each other; an outliner application is a text editor that gives the user control of the visible level of detail of a textual outline, allowing browsing and reorganization of the content according to its structure. Some OPML files may include data concerning size, position and expansion state of the window in which an outline is displayed.
OPML has recently become popular as a format for exchanging subscription lists between RSS feed readers and RSS aggregators. A user can use OPML to track both their own RSS feeds and see who is subscribing, where they are subscribing from and what other feeds other subscribers have chosen.
The standard XML-based OPML editor is open source , which has helped to drive both its growth and adoption. Credit for creating the OMPL protocol itself and driving much of its subsequent growth goes to Dave Winer, the software pioneer instrumental to the creation of XML, RSS 2.0 and many API s (application programming interface) related to Web publishing.
Much like HTML , OPML was designed to be quickly understood and applied with a minimum of technical knowledge. Because the standard is based upon XML, the technology is application and platform-independent and can be adapted for use within academic, scientific or business contexts.