A search engine that queries other search engines and then combines the results that are received from all. In effect, the user is not using just one search engine but a combination of many search engines at once to optimize Web searching.

I never knew about this, probably i may have use it.
By the way good way to define it.

A metasearch engine is a search tool that sends user requests to several other search engines and/or databases and aggregates the results into a single list or displays them according to their source.Metasearch engines create what is known as a virtual database.

Metasearch engines

DeeperWeb
Dogpile
Excite
Harvester42
HotBot
Info.com
Ixquick
Kayak
Mamma
Metacrawler
MetaLib (Not a public search engine)
Mobissimo
SideStep
WebCrawler
Crawler.com