Dec. 14 (Bloomberg) -- Google Inc., owner of the most- popular Internet search engine, started a Web site that will compete with online encyclopedia Wikipedia, where users contribute and edit entries.

The site is called Knol, which stands for a unit of knowledge, the Mountain View, California-based company said yesterday on its blog. Google invited a selected group of users to start testing the site this week.

Wikipedia was created in 2001 and grew to become the eighth most-visited U.S. Web site, according to data for October from ComScore Inc. In challenging Wikipedia, Google is taking on a site that claims 75,000 active contributors working on 9 million articles in more than 250 languages.

``Our goal is to encourage people who know a particular subject to write an authoritative article about it,'' Udi Manber, a Google vice president, wrote on the blog. ``A Knol on a particular topic is meant to be the first thing someone who searches for this topic for the first time will want to read.''

Wikipedia benefits from Google's search engine, which is designed to send users to the sites most relevant to their query. For example, a Google search for Barack Obama, the Democratic presidential candidate from Illinois, provides a link to a Wikipedia page just below the link to his own Web site.

Keeping Users

Google's product may allow the company to keep more users on its own site, the second most-visited in the U.S., rather than sending them to Wikipedia and elsewhere. Google gets 99 percent of its more than $10 billion in annual sales from online advertising, mostly by selling sponsored text links on its own pages and partner sites.

``We welcome the Google Knol initiative,'' Wikipedia, which is owned by the nonprofit Wikimedia Foundation, said in an e- mailed statement. ``The more good free content, the better for the world.''

Google had 131.6 million U.S. visitors in October, more than double Wikipedia's 56.1 million, Reston, Virginia-based ComScore said. Yahoo! Inc., based in Sunnyvale, California, had 136.8 million.

The Wikimedia Foundation, started in 2003, said this year that it's moving to San Francisco in January from St. Petersburg, Florida.

Google shares fell $4.09 to $689.96 at 4 p.m. New York time in Nasdaq Stock Market trading. The shares have gained 50 percent this year.

To contact the reporter on this story: Ari Levy in San Francisco at levy5@bloomberg.net

Last Updated: December 14, 2007 16:07 EST