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Meta Search -

Some meta search engines like Metacrawler.com, DogPile.com search among results taken from various search engines (Google, Yahoo, MSN etc.) and then consolidated into a single Search Engine Result Page (SERP)

Meta Tags -

HTML terminology. Meta tags are HTML entities which are placed within the section of a webpage. The most relevant section to search engines, are the Keywords and Description tags. The Keywords tag is used to place emphasis on specific keywords that the author believes is relevant to the webpage. While no longer as important as they once were, these tags are still used by some search engines.

Mirror Sites -

Multiple copies of a site, usually for redundancy and on different servers, are called mirror sites. Registering such sites can be seen as spam by some search engines, and it wise to let only the primary site be crawled.

Misspellings -

People will often misspell words when searching for them. Often, intelligent marketers will also optimise sites for misspellings in order to capture this sort of traffic.