What is the name for the flat, spreading top of a mature thunderstorm?

The flat, spreading top of a mature thunderstorm is called an anvil cloud.

As a cumulonimbus storm grows, its powerful updraft carries air upward until it reaches a stable layer near the tropopause. Unable to rise easily through that layer, the cloud spreads horizontally, creating the characteristic anvil shape. In meteorological classification, this mature form is called cumulonimbus incus; “incus” is Latin for anvil.

The anvil indicates a developed thunderstorm, not a separate cloud genus. It may spread far downwind from the storm tower and can contain ice crystals that produce fibrous, wispy edges. Lightning, hail, heavy rain, downbursts, and occasionally tornadoes may occur in or beneath the broader storm system.

Mammatus clouds are rounded pouch-like formations that can hang beneath an anvil, but they are not the anvil itself. A shelf cloud is a low outflow feature near the storm’s leading edge. Cumulonimbus is the storm cloud genus, while anvil cloud names its flattened upper structure or the incus form.