Plain English Meaning
It is a wobble where the nose swings side to side while the wings rock in time with it. The aircraft is doing two things at once -- yawing and rolling -- in a repeating rhythm rather than settling down on its own.
Full Definition
A coupled lateral-directional oscillation of an aircraft, commonly known as Dutch roll, in which the aircraft yaws and rolls back and forth around its vertical and longitudinal axes at the same time. It typically occurs in aircraft with strong lateral stability (resistance to rolling) relative to their directional stability (resistance to yawing), producing a rhythmic wagging and rocking motion that, if undamped, can persist or grow.
Context Anchor
Seen in aircraft stability discussions, especially when studying Dutch roll and how an airplane responds after a gust, input, or other .