OMDoc is a markup format and data model for Open Mathematical Documents.
OMDoc serves as semantics-oriented representation format and ontology language for mathematical knowledge. Format, Distribution, Examples
The current stable version is OMDoc1.2 (see the OMDoc1.2 Book), but work on OMDoc1.6, the first step towards OMDoc2.0 has begun.
OMDoc.org serves as a community portal for the OMDoc community and as a clearinghouse for information about OMDoc-based projects and extensions to other sciences.