A noindex tag is an HTML meta tag used to instruct search engines not to include a specific web page in their search results. By adding <meta name="robots" content="noindex"> to the head section of a page, website owners can prevent that page from being indexed, which is useful for keeping duplicate, private, or low-value content out of search engine listings. The noindex tag is a key tool in search engine optimization (SEO) for managing which pages are visible to the public through search engines.