Trinity was the codename of the first nuclear test, conducted in New Mexico on July 16, 1945.
The test took place at the Alamogordo Bombing and Gunnery Range in the Jornada del Muerto desert. It tested an implosion-type plutonium device, known as the Gadget, developed by the Manhattan Project.
Scientists needed a full-scale test because the implosion design was complex and its performance could not be established confidently through laboratory calculations alone. The explosion produced a yield commonly estimated at roughly 20 kilotons of TNT and created a fireball visible for many miles.
Trinity was not a bomb used against a city. Its successful result helped establish confidence in the plutonium design later used in the Fat Man bomb dropped on Nagasaki on August 9. The code name Trinity is often confused with Gadget: Trinity named the test, while Gadget named the experimental device.