Physics: Wave Phase

A wave is a repeating, oscillating pattern that varies over time. In the case of audio, it can be air pressure pattern or voltage pattern.
The cycle of a wave refers to one complete oscillation during which the wave pattern repeats itself.
The starting point for measuring a wave cycle is arbitrary. Any wave point can be taken and the cycle ends in the next same consecutive point (examples of wave cycles are shown in the figure below).

Waveforms can be represented as the graphs generated by rotating a circle's radius. This allows any point on the wave to be characterized by the corresponding angular position of that radius (shown in figure below).
The starting point for measuring the angular position of that radius is arbitrary. Any circle point can be taken as the origin, as long as all subsequent measurements use the same starting point.

Consequently there is a relationship between the angle of the circle radius and a certain fraction of the wave.
Interacting with the figure below, it is possible to check that each 360 degrees corresponds to one full wave cycle, or equivalently each 90 degrees corresponds to 1/4 of a full wave cycle.

Wave phase refers to the position of a point within a wave cycle, typically expressed as an angle (degrees or radians), indicating its state relative to a reference point. It describes the stage of oscillation of the wave at a specific point in space and time, with identical phases meaning points are oscillating in sync.
Two waves are said to be "in phase" when their corresponding points (such as crests and troughs) occur at the same position and time. In other words, their oscillations align perfectly, meaning the peaks of both waves happen simultaneously, as do the troughs.
Likewise, the phase difference between two waves at a given point, indicates how far one wave is shifted relative to another. Those waves are said to be "out of phase" between each other and this phase difference is typically measured in angle (degrees or radians).