In 1952's Operation Ivy, what type of object was "Mike"?

In 1952’s Operation Ivy, “Mike” was a hydrogen bomb—a full-scale thermonuclear device. It was the first successful full-scale test of a multi-megaton weapon using the Teller–Ulam staged-fusion design.

The Mike device was detonated on November 1, 1952, at Elugelab Island in Enewetak Atoll, then part of the U.S.-administered Pacific Proving Ground. The explosion produced a yield of about 10.4 megatons of TNT and completely vaporized the island, leaving a crater roughly 1.9 kilometers wide and 50 meters deep.

Unlike later practical nuclear weapons, Mike was enormous and used cryogenic liquid deuterium as its fusion fuel. Maintaining that fuel at extremely low temperatures required bulky equipment, so the device was primarily a proof-of-concept experiment rather than a deliverable battlefield bomb. Its success nevertheless demonstrated that staged thermonuclear explosions could work.

Operation Ivy contained two tests. Mike was the fusion weapon; King, the second shot, was a very large pure-fission bomb. That distinction is a common source of confusion when the two tests are discussed together.