Blackhat

Shortcuts and tricks used by some SEO agencies and consultants to "game the system" so their pages will rank higher than pages on competing websites. Many of these techniques are unpublished, hence the term, blackhat. Over time, Google and other search engines discover these tricks and disallow those pages that utilize them. Sometimes, the search engines (acting as judge, jury and executioner) completely eliminate those pages (or entire domains) from their indexes.

To keep web developers on the straight and narrow, Google publishes their best practices in their Webmaster Guidelines. But it should be noted that Google does not update this publication on a daily basis, so it is possible to run afoul of the G-police without technically violating any of the guidelines.