Pronounced 'flan-jing', the act of mixing two audio signals together, with one being delayed by a small and slowly changing period producing an output signal with peaks and troughs in a harmonic series. Flangers are similar to phasers (phase-shifting, beyond the scope of this definition) however, if you listen intently enough, you can hear the difference.
The flange effect can best be described (to those who never studied wave mechanics) as a jet sound; sounds like a jet aircraft passing by the listener. Phasing on the other hand sounds much more alien and "other-worldly".
While used in all sorts of different music, flangers are most heavily used in psychedelic and dreamy music where strong harmonics and a "wall-of-sound" depth are often sought - used with good reason too, flanging sounds insanely beautiful when you're as high as a kite. ;)
I was listening to Slowdive
's Machine Gun shortly after returning from another galaxy. Coming down, the flanging was simply too beautiful for my human ears, so I cried and reassessed existence.