Plain English Meaning
Using the same strip of pavement both as a runway and as a path for aircraft to drive along on the ground. Pilots taxi on it and also take off and land on it.
Full Definition
An airport operating practice in which a runway is used not only for takeoffs and landings but also as a surface for aircraft to taxi on. This dual role is common at smaller airports that lack a full parallel taxiway, and it is recognized as a contributing factor to runway incursions and runway confusion because aircraft taxiing on the runway must share that surface with arriving and departing traffic.
Context Anchor
Seen in airport layout and runway confusion discussions, especially when reading about why pilots may mistake a runway for a taxi route or enter the wrong surface.