Apple calls it “Private Browsing.”Microsoft calls it “InPrivate.” Google’s new Chrome browser calls it “Incognito.” And yes, practically everyone else calls it “Porn Mode.”

Chrome’s launch on Tuesday confirmed a new feature as a must-have in Web-browsing software: a cloak of invisibility that hides the user’s path around the Web. Incognito browsing, like a similar setting in a new version of Internet Explorer released last week, is designed to erase any trace of the sites you’ve recently visited, wiping away cached pages and browsing history from your hard drive and turning off the browser’s autocomplete function, which can reveal what you’ve recently typed into text boxes.

That private mode can be used for hiding indiscretions in the Web’s red-light district, or, as Google (nasdaq: GOOG -news - people ) innocently suggests, for planning “surprises like gifts or birthdays.”

But such privacy features have an increasingly more important purpose than hiding your tracks from snooping family members. Google’s and Microsoft’s (nasdaq: MSFT - news - people ) new browsing modes don’t just wipe incriminating data from a user’s hard drive; they offer features that shield users from the Web’s ever-more-aggressive behavioral data-gathering by advertisers.

Increased tracking of user behavior online for targeting ads–the subject of a Senate Commerce Committee hearing last July–is one factor driving demand for that privacy cloak, says the Center for Democracy and Technology’s Ari Schwartz. And those demands may open a new front in the browser war. “Competition in this space is clearly growing,” Schwartz says. “As it plays out, it’s a very good thing for user privacy.”

But browsers have yet to agree on just what “privacy” means. Apple’s(nasdaq: AAPL - news - people ) Safari’s “Private Browsing”–pioneered in 2005–is decidedly designed for hiding your tracks from your spouse, not the Web companies collecting data on your online activities. “Private Browsing” doesn’t block cookies–unique identifier files downloaded by your browser that Web sites and advertisers use to follow you from one site visit to the next. To block or delete cookies, users have to toggle those options or manually delete them in another menu.

Google’s slick new browser takes an important step beyond Safari in making stealth browsing easier: When a user opens an “Incognito” tab in Chrome, it not only stops recording history and the words entered into text fields, but also stores all newly-acquired cookies in a temporary folder. As soon as that Incognito tab is closed, its cookies are deleted. That means someone using Google’s stealth setting can navigate some normal sites in one tab with all of his or her identifying cookies intact, while simultaneously browsing another set of sites in stealth. More>>