The codename of the first nuclear weapon test in July 1945 was Trinity.
The Trinity test took place on 16 July 1945 in the Jornada del Muerto desert of New Mexico, as part of the Manhattan Project. At 5:29 a.m. Mountain War Time, the United States detonated an implosion-design plutonium device known as the “gadget.” The explosion demonstrated that the complicated plutonium weapon design would work.
Physicist J. Robert Oppenheimer, director of the Los Alamos Laboratory, assigned the name Trinity. Its precise inspiration is uncertain, though Oppenheimer later indicated that it may have been connected with the religious poetry of John Donne. The test site was later called Trinity Site.
Trinity is often confused with the Manhattan Project, which was the wider wartime program rather than the test’s codename. Fat Man was the similar plutonium bomb used over Nagasaki, while Little Boy was the uranium bomb dropped on Hiroshima. Trinity therefore names the test, not either city attack or the overall project.