Plain English Meaning
Aircraft landing on two parallel runways are arranged so they are not flying right next to each other. Instead, one is always a little ahead of the other, with a set diagonal distance between them.
Full Definition
A spacing technique used during simultaneous dependent approaches to closely spaced parallel runways, in which aircraft on adjacent final approach courses are sequenced so that they are not abeam (side-by-side) at the same point in space. A required minimum diagonal distance is maintained between an aircraft on one final approach course and the aircraft ahead on the adjacent final approach course, typically 1.0, 1.5, or 2.0 nautical miles depending on runway centerline spacing.
Context Anchor
Seen in instrument approach procedures and air traffic control instructions for simultaneous dependent approaches to parallel runways.