Blown Highlights
Bright areas recorded at maximum value across all channels, rendering as featureless white that no processing can restore.
In depth
Some blown highlights are acceptable and even desirable — a specular reflection on chrome, the disc of the sun, a bare bulb in frame. The problem is diffuse highlights: skin, cloud, fabric, snow. These carry texture the viewer expects to see, and their absence reads immediately as a technical failure.
Examples in practice
Clouds rendered as a flat white shape rather than a modelled form is the classic sign of a blown sky.