Definitions for "Extensible"
Capable of being extended, whether in length or breadth; susceptible of enlargement; extensible; extendible; -- the opposite of contractible or compressible.
Having the potential to be expanded in scope, area or size. In the case of Dublin Core, the ability to extend a core set of metadata with additional elements.
Capable of being extended. A program or system that can be added to and modified in the future.
Provides the X in the acronyms XML and XHTML. Means transformable.
The ability of a specification to accept extensions in a defined way. A specification is extensible if it provides a mechanism for any party to create extensions.
markup language (XML): A widely accepted way of sharing information over the Internet in a way that computers can use, regardless of their operating system.
Keywords:  tags, fields, elements, defined, end
Tags (fields/data elements) can be defined by the end user