What is Cloaking?

Showing different content to Google than to regular site visitors, which is a violation of Google guidelines and can result in a manual action penalty.

From How to Recover from 5 Common Google Penalties by Aaron Aglus #352

An SEO violation where a website shows a different page to users than to search engine bots; the episode notes Google banned its own AdWords pages for this practice.

From We discuss the top 10 SEO mistakes - Unknown Secrets of SEO E-Webstyle Number 65

A technique where server code detects search engine bots and serves them a different version of a webpage than human visitors see, which violates Google guidelines and can result in a site ban.

From Find out how to use a keyword density tool as which meta tags are important to Google - Unknown Secrets of SEO E-Webstyle Number 27

Showing one version of page content to search engine robots and a different version to human users; flagged as a negative ranking factor when done with malicious or manipulative intent.

From Search Engine Visitor Optimization (SEVO) and Google Factors for SERPS - #seopodcast 43

Showing different content to the Googlebot than to regular users, identified by user agent or IP address. Google considers this a violation and may ban the site.

From GoWalla, Panda Farmer and SEO - Internet Marketing Podcast 100