The term "Phase" is used to express a location in time relative to the origin of a cyclical waveform.
In trigonometry, a "Sine" wave is derived from plotting the "Y" value of points on a circle of radius=1 centered at X=0, Y=0 on a graph with the number of degrees of rotation along the "X" axis.
- The sine wave "starts" at "X=0 degrees, Y=0" and passes through "Y=1" at 90 degrees. The sine wave then passes back through "Y=0" at 180 degrees completing the positive half-cycle of the waveform.
- The sine wave continues and passes through "Y= -1" at 270 degrees, and continues until it reaches "Y=0" again at 360 degrees completing both the negative half-cycle and the entire sine wave.
In audio, at 360 degrees the process starts over and continues until the audio source stops outputting a signal. How long each sine wave takes to complete a cycle is the "period" of the cycle and the inverse (or 1/period) is the frequency in cycles per second (c.p.s.); which is also referred to as "Hertz."
If two sine waves of the same frequency are perfectly aligned in time; they are said to be "in-phase" so that at any point in time, the sine waves are at the same part of their cycle (at the same number of degrees). Any change in the time relationship between the two waveforms will make them "out-of-phase" by a number of degrees.
If the two waveforms are 180 degrees out-of-phase one sine wave is in the positive half-cycle when the other is in the negative half-cycle; and if added together ("mixed" or "summed" in audio terms); they will cancel each other completely. Adding two sine waves of opposite polarity has the same effect- they will cancel each other completely. This is why polarity is sometimes referred to in a confusing manner as "absolute phase."
One other common example of when the term "phase" is used to describe a polarity issue is when someone connects stereo speakers incorrectly, resulting in less bass than normal. Most people would say the speakers were "out-of-phase" with each other because the incorrect polarity of one speaker connection results in the acoustic subtraction of the bass from the two speakers due to "phase cancellation," instead of the bass adding as it would with the speakers both connected with the correct polarity.
For more information; see waveform.