Look up validator in Wiktionary, the free dictionary.

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,^[1][2] CSS, and XML documents like RSS feeds, though it can be used for any defined format or language.

Accessibility validators are automated tools that are designed to verify compliance of a web page or a web site with respect to one or more accessibility guidelines (such as WCAG, Section 508 or those associated with national laws such as the Stanca Act).