Meta Search Engine
A type of search engine. A website that takes your search query and passes it on to several different search engines and directories, then summarizes the results in a logical manner for you to review.

Meta Tag
An HTML tag placed in the head section of a web page. The tag provides additional information that is not displayed on the page itself. The initial idea was that webmasters should use these tags to help search engines index the page correctly by providing an accurate description of the page content and a list of keywords associated with the page. Unfortunately this left the door open to abuse. Many webmasters used these tags to gain an unfair advantage, forcing search engines to begin disregarding meta tags.

Mirror Sites
Identical, but separate websites on different domains. They are commonly used legitimately by large websites to share heavy server loads, and by search engine spammers to generate more search engine referrals and revenue.

In general, the search engines frown upon mirror sites and do not hesitate to assess duplicate content penalties when they feel they are warranted.