Apple is defending its effort to trademark the phrase "App Store" against a Microsoft challenge, arguing the term is no more generic than the software giant's trademarked "Windows."

Apple applied for the trademark in 2008, prompting a challenge by Microsoft, which contended the phrase is too generic to register and would restrict competitors' ability to describe their own services. In a filing (PDF) yesterday with the U.S. Patent and Trademark Office, Apple asked that the challenge be dismissed and pointed out that one of Microsoft's most prominent trademarks has also been challenged as generic.

"Having itself faced a decades-long genericness challenge to its claimed Windows mark, Microsoft should be well aware that the focus in evaluating genericness is on the mark as a whole and requires a fact-intensive assessment of the primary significance of the term to a substantial majority of the relevant public," says Apple in the filing (Apple's emphasis). "Yet, Microsoft, missing the forest for the trees, does not base its motion on a comprehensive evaluation of how the relevant public understands the term App Store as a whole."

Apple's trademark application for "app store" was filed a week after the company launched its iPhone App Store, and described a retail store offering "services featuring computer software provided via the Internet and other computer and electronic communication networks," as well as other services.

Microsoft filed a motion in January opposing the application with the agency's Trademark Trial and Appeal Board, saying "app store" is "generic for retail store services featuring apps and unregistrable for ancillary services such as searching for and downloading apps from such stores." Microsoft's filing cited efforts to trademark "The Computer Store" and "Log Cabin Homes," applications the board rejected for their generic natures.

Microsoft's motion pointed out that the phrase has become common enough that Apple CEO Steve Jobs used the phrase during an interview last October to criticize the proliferation of similar efforts from competing companies.

However, Apple argued in its filing that the term isn't generic and pointed out that other companies with competing online enterprises offering apps for mobile devices have found alternatives to the App Store moniker:

In limited instances third parties have made improper use of the term App Store. In response, Apple has contacted those parties and requested that they cease and desist from further use of the mark. In most every instance, the entities contacted by Apple agreed to cease use of Apple's App Store mark. Those few which refused to cease use of Apple's App Store mark made reference to Microsoft's challenge of Apple's rights in its App Store mark, which has received widespread attention in the press, and have refused to cease using App Store pending a ruling in this proceeding.

Microsoft said today it sticks by its argument and will continue its challenge of Apple's trademark application.

"We remain confident in our case and believe that the term 'App Store' should continue to be available for use by all without fear of reprisal by Apple," a Microsoft representative told CNET. "We will continue to move forward with the trademark opposition proceeding in the Trademark Trial and Appeal Board."

Apple representatives did not immediately respond to a request for comment.

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Steven Musil is the night news editor at CNET News. Before joining CNET News in 2000, Steven spent 10 years at various Bay Area newspapers. E-mail Steven.