President Obama has ordered the closure of Guantanamo Bay
The US is abandoning its use of the term "enemy combatants" to describe terror suspects - ending a key policy of the Bush administration.
It is the latest shift on Guantanamo Bay by President Barack Obama, who has announced the camp is to be closed.
The decision to drop the term is deeply symbolic, correspondents say.
President George W Bush argued that his status as commander-in-chief allowed him to hold "enemy combatants" indefinitely and without trial.
Announcing the end of the term's use, the Justice Department said suspects would in future be held according to legal standards set by the international laws of war.
Under the new definition, only those who provided "substantial" support to al-Qaeda or the Taleban will be considered detainable, officials said.
By using the term "enemy combatants", the Bush administration argued that they were not prisoners of war, the BBC's Jonathan Beale in Washington says. International laws - like the Geneva Conventions - therefore did not automatically apply.
The Obama administration will, by contrast, hold prisoners under the authority granted by Congress, when it approved the Authorisation for the Use of Military Force "against nations, organizations, or persons the president determines planned, authorized, committed, or aided the September 11 attacks, or harboured such organizations or persons" in September 2001.
"The government's new standard relies on the international laws of war to inform the scope of the president's authority under this statute," according to the Justice Department.
"As we work towards developing a new policy to govern detainees, it is essential that we operate in a manner that strengthens our national security, is consistent with our values, and is governed by law," said US Attorney General Eric Holder in a statement.
Around 250 detainees are still being held in Guantanamo, our correspondent says.
The Obama administration is currently reviewing each case before making a decision as to who should stand trial, he adds.
The detention centre was set up in January 2002 by the Bush administration to hold men captured in the fight against al-Qaeda and the Taleban after the 2001 attacks on the US.
It later became notorious because of the harsh interrogation techniques used there, which many said amounted to torture, and because the detainees there were being held without trial and without many of the protections of international law.
During the presidential election, Mr Obama pledged to close the camp if elected, and in his first week in the White House, he issued the order to do so.
Some of the detainees are expected to stand trial in US courts, while others will be returned to their home countries.
But the Obama administration is likely to face difficulties prosecuting some inmates, because evidence against them may have been obtained using inadmissible techniques.
Other prisoners cannot be sent back to their home countries because they may face torture or execution if they return.
Mr Obama this week appointed diplomat Dan Fried to serve as a special envoy to persuade third countries to accept detainees who cannot be extradited to their home countries.