- Rapid variations in received signal levels, such as variations that may be caused by atmospheric disturbances, antenna movements in a high wind, or interaction with other signals.
- In radio propagation, a phenomenon in which nearly all radio signals that are usually reflected by ionospheric layers in or above the E-region experience partial or complete absorption.
- In radio transmission, rapidly changing signal levels, together with variable multipath time delays, caused by reflection and possible partial absorption of the signal by aircraft flying through the radio beam or common scatter volume.
- The variation in the transmission characteristics of a loaded telephone line caused by the action of telegraph direct currents on the loading coils.
- In recording and reproducing equipment, the deviation of frequency caused by irregular mechanical motion, e.g., that of capstan angular velocity in a tape transport mechanism, during operation.
In the field of mechanics and structures, Aeroelastic flutter is an aeroelastic phenomenon where a body's own aerodynamic forces couple with its natural mode of vibration to produce rapid periodic motion. Aeroelastic flutter occurs under steady flow conditions, when a structure's aerodynamic forces are affected by and in turn affect the movement of the structure. This sets up a positive feedback loop exciting the structure's free vibration. Flutter is self-starting and results in large amplitude vibration which often lead to rapid failure.
The aerodynamic conditions required for flutter vary with the structure's external design and flexibility, but can range from very low velocities to supersonic flows. Large or flexible structures such as pipes, suspension bridges, chimneys and tall buildings are prone to flutter. Designing to avoid flutter is a fundamental requirement for rigid airfoils (fixed wing aircraft and helicopters) as well as for aircraft propellers and gas turbine blades.
Prediction of flutter prior to modern unsteady computational fluid dynamics was based on empirical testing. As a result, many pioneering designs failed due to unforeseen vibrations. The most famous of these was the opening of the original Tacoma Narrows Suspension Bridge in mid 1940, which failed spectacularly 4 months later during a sustained 67 km/h crosswind and became known as Galloping Gertie for its flutter movement.
During the 1950s over 100 incidents were recorded of military or civilian aircraft being lost or damaged due to unforeseen flutter events. While as recently as the 1990s jet engine flutter has grounded military aircraft.
Techniques to avoid flutter include changes to the structure's aerodynamics, stiffening the structure to change the excitation frequency and increasing the damping within the structure.