A metasearch engine is an Internet search system that supports unified access to multiple search engines through one search query.

A search engine visits billions of websites, using an automated program known as a spider. Information collected by these spiders is stored in a database known as an index. When a user enters a search query into a search engine, such as Google or Bing, an algorithm will determine which of the pages contained in its index are the most relevant to the search terms used in the query. Web pages that have not been included in the search engine's index will not be available in its search engine results.

Metasearch engines don't send their own spiders out to search the Web, and they don't have an index of their own to draw from. Instead, a metasearch engine will pull results from a number of search engines, and then apply their own algorithms to reorder the results. To those who visit a search engine site or a metasearch site, the two will most often appear to be the same, generally a search field in the center of a large white space. Technically, a metasearch engine is not a search engine, but a web portal that aggregates search engine results through a proprietary algorithm.

The same search query will produce different results from one search engine to another. This is because they are using different indexes and each has its own proprietary algorithm. When the same search query is made on two different metasearch engines, the results will also differ because, of course, they each use their own proprietary algorithms, but also because the two metasearch engines may use data from different search engines.

When a user enters a search term into the search field of a metasearch engine and begins a search, the engine will send the request to multiple search engines. Since the metasearch engine is under contract to pay the search engines whose results they use, a metasearch engine that uses both Google and Bing will have access to a much larger pool of data than one that uses data from smaller search engines.

After the metasearch engine sends the query to the search engines that it has access to, its server will wait for the responses from each requested search engine before displaying search results to the user. In some metasearch engines, the results pages continue to be updated as results from other search engines come in.

The order of the results that are displayed is determined by the metasearch engine's individual algorithm. Results may be compiled based, in part, to the popularity of the requested search engines. Thus, Google results may be given precedence over a lesser known engine. Most metasearch engines also filter out duplicates so that the URL does not appear twice in the results that are displayed.

Some metasearch engines allow the user to select the search engines to be used for their search.

Some search engines maintain their own indexes as well as incorporating results from other sources. For example, Yahoo has its own search engine spider and maintains its own index, but it also uses results from Bing. Prior to entering into a contract with Bing, it used Google results for a time. For the sake of categorization, the Aviva Directory will be listing them as search engines, not metasearch engines.

Because even the largest of search engines have indexed only a small portion of the Web, a metasearch engine will provide access to a larger portion of the Internet than a search engine, as it can be assumed that individual search engines have indexed different parts of the Web, although there will be many similarities in results.

We might assume that, because metasearch engines get their results from multiple search engines, the results they provide will be more helpful than those of any one individual search engine. In reality, it's a matter of personal preference.

If each individual search engine indexed a completely different part of the Web, then a metasearch engine that is able to bring them all together would be a better choice than any one search engine. In reality, it seems that there are more similarities than there are differences in the content provided in the indexes of the various search engines. Because there are some differences, you might find that one search engine will be more efficient for a particular search while another might yield better results in another type of search.

Probably, the differences between individual search engines and metasearch engines have more to do with the algorithms they use to order their results than in the pages that are included in their indexes.

In the early 2000s, the consolidation of the search engine market resulted in several once popular search engines being converted into metasearch engines, and some of these have gone back and forth a few times and may continue to do so.

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Search for Metasearch Engines on Google or Bing