Phase

Phase refers to the orientation of an audio waveform. If you consider a waveform as a mapping of energy above and below a zero voltage line, the phase modulates back and forth between positive and negative voltages. Eventually, these modulations get reproduced by a speaker, which vibrates inward (negative voltage) and outward (positive voltage), thereby moving air to make sound. Any given modulation can be either positive or negative voltage, depending on its phase.