As recently as a few weeks ago, “metadata” was an obscure term known mainly to techies and academics. Broadly defined, metadata is data about other data. For the phone company, it might be the time and length of your calls, but not the conversation itself; in the context of e-mail, it means information such as the sender and recipients of a message—basically, everything except what the message actually says.
Then came the revelation that the National Security Agency has been collecting metadata about millions of Americans’ phone calls. Suddenly metadata exploded as a public issue. Is it a harmless way for the government to track dangerous patterns or a tightening net around our lives?