1an act of fluttering:there was a flutter of wings at the window a state or sensation of tremulous excitement:her insides were in a flutter [mass noun] Medicine disturbance of the rhythm of the heart that is less severe than fibrillation. [mass noun] Aeronautics undesired oscillation in a part of an aircraft under stress. [mass noun] Electronics rapid variation in the pitch or amplitude of a signal, especially of recorded sound. Compare with wow2.
2British informal a small bet:a flutter on the horses
(of a bird or other winged creature) fly unsteadily or hover by flapping the wings quickly and lightly:a couple of butterflies fluttered around the garden [with object] (of a bird or other winged creature) flap (its wings) quickly and lightly. move with a light irregular or trembling motion:flags of different countries fluttered in the breeze (as adjective fluttering)a fluttering banner [with adverbial of direction] (of a person) move restlessly or uncertainly:Mavis fluttered about nervously (of a pulse or heartbeat) beat feebly or irregularly.