Definition: A method of connecting to the Internet or a particular web site and reporting a different IP Address than your assigned one. IP Spoofing is used in SEO in order to trick a spider into indexing one site, while actually presenting another site to the end user. This tactic can also be used to redirect a user to a site other than the one they have clicked on.
This tactic is considered highly unethical and is illegal in some areas.
Source:
http://websearch.about.com/library/g...p-spoofing.htm