nofollow is a value that can be assigned to the rel attribute of an HTML a element to instruct some search engines that a hyperlink should not influence the link target's ranking in the search engine's index. It is intended to reduce the effectiveness of certain types of internet advertising because their search algorithm depends heavily on the number of links to a website when determining which websites should be listed in what order in their search results for any given term. However, new information suggests that search engines will depend less on links in the future and more on other aspects such as mentions that don't include links.
The nofollow value was originally suggested to stop comment spam in blogs. Believing that comment spam affected the entire blogging community, in early 2005 Google's Matt Cutts and Blogger's Jason Shellen proposed the value to address the problem.
The specification for nofollow is copyrighted 2005–07 by the authors and subject to a royalty free patent policy, e.g. per the W3C Patent Policy 20040205, and IETF RFC 3667 & RFC 3668. The authors intend to submit this specification to a standards body with a liberal copyright/licensing policy such as the GMPG, IETF, and/or W3C.
Google announced in early 2005 that hyperlinks with rel="nofollow" would not influence the link target's PageRank. In addition, the Yahoo and Bing search engines also respect this attribute value.