What is Grey hat SEO?

Gray Hat SEO is the practice of tactics/techniques which remain ill-defined by published material coming out of Google, and for which reasonable people could disagree on how the tactics support or contrast with the spirit of Google’s published guidelines.
It is a computer hacker or computer security expert who may sometimes violate laws or typical ethical standards, but does not have the malicious intent typical of a black hat hacker. They might be legal today and get you high, but in a couple of years time you could get punished for using them. Grey hat SEO involves much more creativity than Black hat SEO. The reason Grey hat SEO is less known about is that as soon as the search engines become aware that people are using these techniques, they take steps to prevent them, which turns a grey hat technique into a black hat technique.