Death's flag is the flag flying above Old Glory when the nation is in mourning. No, you can't see it, but at least you're thinking about it, and that's the whole point of the American flag being at half mast.
The tradition dates back to the 1612, when the British ship Heart's Ease arrived in Canada with her captain dead. When it next arrived in London, its Union Jack was at half mast, making room for the invisible flag of death.
The U.S. Navy first observed the custom in 1799 to mark the death of George Washington. The Navy Department ordered U.S. Navy ships to "wear their colours half mast high." The country would follow suit after that, but no guidelines were given for when and for whom it was appropriate.
Title 4, Chapter 1, Section 7 of the United States Code outlines strict guidelines for flying the U.S. flag, and for lowering it, depending on who died. All Presidents are remembered for thirty days while the current Vice-President, Chief Justice of the Supreme Court, and Speaker of the House get ten days. The Department of Veterans Affairs has a handy quick reference page for flying the flag at half mast, adding "The flag should be briskly run up to the top of the staff before being lowered slowly to the half-staff position."
President Eisenhower declared structure for lowering the flag in 1954. the President can order the flag lowered to mourn the deaths of other officials and foreign dignitaries as well as to mark tragic events in the history of a nation. And no, President Obama did not order the flag at half mast for Whitney Houston.
The flag was lowered nationally for Pope John Paul II, Neil Armstrong, Rosa Parks, Winston Churchill, Anwar Sadat, Yitzhak Rabin and Nelson Mandela. It was also lowered to mourn the shootings in Virginia Tech, Newtown, Conn., and Charleston, as well as for the Boston Marathon Bombing and the Indian Ocean Tsunami in 2004.
Governors of the states, territories, and the Mayor of Washington, D.C. also have the authority to lower the flags in areas under their jurisdiction.
If you can't lower you flag because its in a fixed position on the pole, the American Legion advises you to put a black ribbon to the top of the pole.