When pages are removed from a search engine's index specifically because the search engine has deemed them to be spamming or violating search engine guidelines.
Also called a doorway page. A gateway page exists solely for the purpose of driving traffic to another page. They are usually designed and optimised to target one specific keyphrase. Gateway pages rarely are written for human visitors. They are written for search engines to achieve high rankings and hopefully drive traffic to the main site. Using gateway pages is a violation of most search engine guidelines and could be grounds for banning.
Text that is visible to the search engines but is invisible to humans. It is primarily used for the purpose of including extra keywords in the page without distorting the aesthetics of the page. Using hidden text is a violation of search engine guidelines and can be grounds for banning.
Guidelines provided by search engines to improve the ranking of web site. The guidelines describe the illicit practices that may lead to a site being banned. Link to Google guidelines.