Plain English Meaning

When air speeds up and spreads out as it flows around the aircraft, it gets a little colder in places. That extra cooling can drop a wing or other surface below freezing, so water droplets striking it can turn into ice.

Full Definition

A reduction in the temperature of a surface or airflow caused by the rapid expansion or low-pressure conditions produced as air moves over an aircraft. In icing contexts, it refers to the local cooling that occurs on airframe surfaces where airflow accelerates and pressure drops, allowing supercooled water droplets to freeze on impact even when the ambient air temperature is slightly above freezing.