A validator is a computer program

Computer program

A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...

 used to check the validity

Validity

In logic, argument is valid if and only if its conclusion is entailed by its premises, a formula is valid if and only if it is true under every interpretation, and an argument form is valid if and only if every argument of that logical form is valid....

 or syntactical correctness of a fragment of code or document. The term is commonly used in the context of validating HTML

HTML

HyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....

, CSS

Cascading Style Sheets

Cascading Style Sheets is a style sheet language used to describe the presentation semantics of a document written in a markup language...

 and XML

XML

Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....

 documents or RSS

RSS (file format)

RSS is a family of web feed formats used to publish frequently updated works—such as blog entries, news headlines, audio, and video—in a standardized format...

 feeds though it can be used for any defined format or language.

HTML validator

In the context of web

World Wide Web

The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...

 documents, the term HTML validator technically refers to validation using DTDs

Document Type Definition

Document Type Definition is a set of markup declarations that define a document type for SGML-family markup languages...

 by SGML tools. However, the common (non technical) use of the term refers to checking an HTML

HTML

HyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....

 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

HTML 5

HTML5 is a language for structuring and presenting content for the World Wide Web, and is a core technology of the Internet originally proposed by Opera Software. It is the fifth revision of the HTML standard and is still under development...

cannot mean validation using a DTD because HTML 5 does not use a DTD.