A definition provides a formal skill and conceptual oriented information on a concept managed by the documentation.
The term "SchemaDoc" is used to describe three kinds of real world objects : a project, a format and a publishing tool.
As a project, SchemaDoc tends to accomplish the goals it's been assigned to. There's no functional upper limit to what is provided as the project : development facilities and tools, mailing lists. The lower limit is the availability, by all means, of the format and the source code of a tool.
As a format, SchemaDoc is an implementation of a documentation model. It is provided as a documented XML Schema, the documenting part being recursively implemented in SchemaDoc format itself.
As a tool, SchemaDoc is aimed at publishing SchemaDoc documents, not writing them, not even editing their textual components. It comes as :
a set of Java packages and XML files developed by the SchemaDoc project,
the SchemaDoc format,
and a collection of freely available XML files (DTD, stylesheet, etc.) provided for compatibility and convenience purpose.