Supreme Court Justice Potter Stewart famously said that he couldn't define obscenity, but that he knew it when he saw it.

The same has long been the case with spyware. It's not easy to define, but most people know it when parasitic programs suck up resources on their computer and clog their browsers with pop-up ads.

Recognizing that one person's search toolbar is another's spyware, a coalition of consumer groups, ISPs and software companies announced on Tuesday that it has finally come up with a mutually agreeable definition for the internet plague.

Spyware impairs "users' control over material changes that affect their user experience, privacy or system security; use of their system resources, including what programs are installed on their computers; or collection, use and distribution of their personal or otherwise sensitive information," according to the Anti-Spyware Coalition, which includes Microsoft, EarthLink, McAfee and Hewlett-Packard.

The group hopes the definitions will clear the way for anti-spyware legislation and help create a formal, centralized method for companies to dispute or change their software's classification.

"One of the biggest challenges we've had with spyware has been agreeing on what it is," said Ari Schwartz, associate director of the Center for Democracy and Technology, which has led the group's work. "The anti-spyware community needs a way to quickly and decisively categorize the new programs spawning at exponential rates across the internet."

The lack of standard definitions of spyware and adware has doomed federal and state legislation and hampered collaboration between anti-spyware forces.

In a colloquial sense, spyware is used to refer to a whole range of programs, including unwanted browser toolbars that come bundled with other downloads, surf-tracking software that generates pop-up ads, and software that tries to capture passwords and credit-card numbers.

Software companies like Claria, which distribute their pop-up advertising software by bundling it with free programs such as peer-to-peer software, adamantly deny their products are "spyware." They point out that users can usually find a definition of the programs' effects deep in the user agreement.

It is unclear what effect the new definitions will have on current anti-spyware programs, such as Lavasoft's Ad-Aware and Microsoft's free AntiSpyware tool.

Recently, Microsoft downgraded the default program action for Claria's software from "Remove" to "Ignore," which prompted widespread criticism.

Microsoft responded by saying that it had changed the handling of "Claria software in order to be fair and consistent with how Windows AntiSpyware (beta) handles similar software from other vendors."

Microsoft is in negotiations to buy venture-capital-backed Claria, according to The New York Times.

Ben Edelman, the country's foremost spyware researcher, questions whether the new definitions are simply there so that adware companies can find a way to get a stamp of approval for their software.

"From the perspective of users whose computers are infected, there is nothing hard about (defining spyware)," Edelman said. "If you have adware or spyware on your computer, you want it gone.

"Maybe the toolbar is Mother Theresa, but it's Mother Theresa sitting in your living room uninvited and you want her gone also," Edelman said. "You don't need a committee of 50 smart guys in D.C. sipping ice tea in order to decide that.

"The question is, what do you want to do with it? If you had a consensus of 100 computer-repair technicians or Bill Gates himself, what would they say to do?"