Noindex
The noindex directive asks search engines not to retain a page in their index. It is placed in the page’s meta tags or HTTP header, and requires that the page remain crawlable so the directive can be read.
In practice
Legitimate uses: filter pages, internal search results, duplicate content useful to users but not to engines. A classic mistake: blocking the page in robots.txt while expecting the noindex to work — since the robot cannot read the directive, the page may remain indexed. Prolonged noindex also eventually discourages crawling of the page altogether.