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.

References

1. W3C Markup Validation Service
2. W3C CSS Validation Service
3. WAVE web accessibility evaluation tool
4. Variety of checks
5. Backwards Compatibility Checker