Validator

A validator is a computer program used to check the validity or syntactical correctness of a fragment of code or document. The term is commonly used in the context of validating HTML, CSS and XML documents or RSS feeds though it can be used for any defined format or language.

HTML Validator
In HTML context, the term HTML Validator technically refers to validation using DTDs by SGML tools. However, the common (non technical) use of the term HTML Validator means to check an HTML document for various problems using some type of computerized checker, and not necessarily by using a DTD with SGML tools. Furthermore, the term HTML Validation for HTML 5 cannot mean validation using a DTD because HTML 5 does not use a DTD.