Pilot-induced oscillation

Pilot-induced oscillations (PIOs), as defined by MIL-HDBK-1797A, are sustained or uncontrollable oscillations resulting from efforts of the pilot to control the aircraft. They occur when the pilot of an aircraft inadvertently commands an often increasing series of corrections in opposite directions, each an attempt to cover the aircraft's reaction to the previous input with an overcorrection in the opposite direction.

Source: Wikipedia — Pilot-induced oscillation (CC BY-SA 4.0)