Plain English Meaning
It happens when a cold front catches up to a warm front, and the air pushing in behind is even colder than the air it meets. That coldest air slides underneath, shoving the warm air up and out of the way and leaving a single boundary on the surface between the two cooler air masses.
Full Definition
A type of occluded front that forms when a fast-moving cold front overtakes a slower-moving warm front, and the air behind the cold front is colder than the cool air ahead of the warm front. The colder, denser air wedges under the cool air, lifting the warm air aloft and producing a mix of cold front and warm front weather characteristics along the boundary.
Context Anchor
Seen in aviation weather study, weather briefings, and surface weather charts when learning how occluded form and what weather they can bring.