Trinity was the code name of the first detonation of a nuclear weapon, conducted by the United States Army on July 16, 1945, as a result of the Manhattan Project. The new test site, named the White Sands Proving Ground, was built in the Jornada del Muerto desert about 35 miles (56 km) southeast of Socorro, New Mexico, at the Alamogordo Bombing and Gunnery Range (now part of the White Sands Missile Range).

Trinity used an implosion-design plutonium device, informally nicknamed "The Gadget" or "Christy['s] Gadget" after Robert Christy, the physicist behind the implosion method used in the device. Using the same conceptual design, the Fat Man device was detonated over Nagasaki, Japan, on August 9, 1945. The Trinity detonation produced the explosive power of about 20 kilotons of TNT (84 TJ).

Although nuclear chain reactions had been hypothesized in 1933 and the first artificial self-sustaining nuclear chain reaction (Chicago Pile-1 or CP-1) had taken place in December 1942, the date of the Trinity test is usually considered to be the beginning of the Atomic Age. It is also called Ground Zero where the first atomic bomb was detonated.

In 1952, the site of the explosion was bulldozed, and the remaining trinitite was disposed of. On December 21, 1965, the 51,500-acre (20,800 ha) area Trinity Site was declared a National Historic Landmark district and, on October 15, 1966, was listed on the National Register of Historic Places.

The landmark includes the base camp, where the scientists and support group lived; ground zero, where the bomb was placed for the explosion; and the Schmidt/McDonald ranch house, where the plutonium core to the bomb was assembled. Visitors to a Trinity Site open house are allowed to see the ground zero and ranch house areas. In addition, one of the old instrumentation bunkers is visible beside the road just west of ground zero.