Outline Processor Markup Language (OPML)

Definition - What does Outline Processor Markup Language (OPML) mean?

Outline Processor Markup Language (OPML) is an open-source XML format for creating text outlines. OPML is platform-independent, can handle many types of data and may be customized for each application created. It is particularly suited to creating applications where relationships and data must be updated continually. The format is human-readable, self-documenting and extensible. Some OPML files contain data specifying the size, position and expansion capabilities of the windows in which the text outlines are displayed.

OPML can be quickly understood and applied, much like HTML. Because it is based on XML, OPML can be adapted to business, scientific or academic projects.

Techopedia explains Outline Processor Markup Language (OPML)

Outline Processor Markup Language has evolved into a format used for exchanging subscription lists between RSS aggregators and RSS feed readers. Users can track their own RSS feeds as well as observe who is subscribing, where they are from and the feeds they have chosen.

OPML also has some shortcomings:

The date format only allows two-digit years and the format does not conform to RFC 3339.
The expansion state of some windows cannot be stored.
When a window is altered or deleted, the windows below it must be recalculated.
The arbitrary nature of the type attribute, and the use of arbitrary attributes on outline elements causes the interoperability of the documents produced to be almost completely dependent on the conventions of the content producers, which may be neither standard nor documented.
There are problems with identifying created documents as XML format.