1) The use of the Robots Exclusion Protocol (REP) in a site’s robots.txt fie to block web robots (bots) and search engine indexing for specific pages, directories or the entire site. Honoring the REP is voluntary. While major search engines will honor the request, other bots may still crawl the pages.
2) The decision by a user to block pages from certain websites in their search results. If a site is blocked by a large number of users its search engine rankings can be negatively affected.