A large cloud of rubble and dust shaped like a mushroom and rising into the sky after an explosion (especially of a nuclear bomb) WordNet 3.0 & Open English WordNet

A mushroom cloud is a distinctive mushroom-shaped flammagenitus cloud of debris, smoke, and usually condensed water vapor resulting from a large explosion. Wikipedia

Mushroom-shaped cloud produced by a large explosion Wikidata

A large cloud of smoke, flame and debris that rises into the sky as a result of a large explosion, especially after a nuclear explosion, characterized by a long shaft rising from the ground into a much larger or more spread out top, reminiscent of a mushroom in shape. Wiktionary

Mushroom shaped cloud. Wiktionary (translation)