Flanger

From AudioLexic

An digital delay based effect which makes a copy of the input signal and delays it slightly (between about 1 and 10 milliseconds), inverts the phase, and sweeps the frequency. The resulting sweeping sound is caused by frequency cancellation.

The term dates back to the days when the effect was created by recording the sound onto two identical tape recorders while the engineer applied pressure to one of the deck’s flanges (sides of a reel), causing pitch fluctuations.