Final answer:
The W3C specification for defining elements in XML documents is known as XML Schema. It allows for the creation of custom tags and sets rules for structure and data types.
Step-by-step explanation:
The question refers to the W3C specification that outlines how to define elements within an XML document. This specification is called XML Schema. XML Schema is a powerful set of tools for defining the structure, content, and to some extent, the basic data types that XML documents can contain. With it, developers can create custom tags, enforce structure, and set data type constraints for elements and attributes in XML files.