A phaser is an electronic sound processor used to filter a signal by creating a series of peaks and troughs in the frequency spectrum. The position of the peaks and troughs of the waveform being affected is typically modulated by an internal low-frequency oscillator so that they vary over time, creating a sweeping effect.
Phasers are often used to give a “synthesized” or electronic effect to natural sounds, such as human speech. The voice of C-3PO from Star Wars was created by taking the actor’s voice and treating it with a phaser.