Grey Hat SEO
Grey Hat SEO describes the practice of using both black hat and white hat SEO techniques, often in ways that leave room for argument as to whether they're black hat or white hat. Grey Hat SEO is also used to describe any SEO techniques that haven't been judged as black hat or white hat by the major search engines.
In Search Engine Optimization (SEO) lingo, there is black hat SEO, white hat SEO, and grey hat SEO. Black hat SEO techniques are clearly prohibited by search engines (including things like spamming and link schemes, which do increase traffic dramatically but at a big price). Using black hat techniques will get your website banned if you are caught. White hat SEO techniques are approved, ethical methods of increasing traffic and optimizing your website to be search-engine friendly.
Grey Hat SEO is a bit of a misnomer because the name implies the techniques lie somewhere between black and white. However, some grey hat techniques haven't been categorized yet by search engines. Others simply combine the ethical, approved methods with the riskier, banned black hat SEO techniques, hoping they will get away with it.