Washington

If you tweet a picture from your living room using your smartphone, you're sharing far more than your new hairdo or the color of the wallpaper. You're potentially revealing the exact coordinates of your house to anyone on the Internet.

The GPS location information embedded in a digital photo is an example of so-called metadata, a once-obscure technical term that's become one of Washington's hottest new buzzwords.

The word first sprang from the lips of pundits and politicians earlier this month, after reports disclosed that the government has been secretly accessing the telephone metadata of Verizon customers, as well as online videos, emails, photos and other data collected by nine Internet companies.

President Barack Obama hastened to reassure Americans that "nobody is listening to your phone calls," while other government officials likened the collection of metadata to reading information on the outside of an envelope, which doesn't require a warrant.

But privacy experts warn that to those who know how to mine it, metadata discloses much more about us and our daily lives than the content of our communications.

So what is metadata? Simply put, it's data about data. An early example is the Dewey Decimal System card catalogs that libraries use to organize books by title, author, genre and other information.

In the digital age, metadata is coded into our electronic transmissions.

"Metadata is information about what communications you send and receive, who you talk to, where you are when you talk to them, the lengths of your conversations, what kind of device you were using and potentially other information, like the subject line of your emails," said Peter Eckersley of the Electronic Frontier Foundation, a digital civil liberties group.

Powerful computer algorithms can analyze the metadata to expose patterns and to profile individuals and their associates, Eckersley said.

Metadata records of search terms and Web page visits also can reveal a log of your thoughts by documenting what you've been reading and researching, hey said.

"That's certainly enough to know if you're pregnant or not, what diseases you have, whether you're looking for a new job, whether you're trying to figure out if the NSA is watching you or not," he said, referring to the National Security Agency.

Such information provides "a deeply intimate window into a person's psyche," he added.

The more Americans rely on their smartphones and the Internet, the more metadata is generated.

Metadata with GPS locations, for example, can trace a teenage girl to an abortion clinic or a patient to a psychiatrist's office, said Karen Reilly, the development director for The Tor Project, a U.S.-based nonprofit that produces technology to provide online anonymity and circumvent censorship.

Metadata can even identify a likely gun owner, she said.

"Never mind background checks — if you bring your cellphone to the gun range you probably have a gun," Reilly said.