Phase means how far along you are in a cycle of something periodic. In EE, that’s usually a cycle of alternating current. For reasons related to the math of circles, we measure phase in radians, which is usually an angle measurement. 2 pi radians is an angle all the way around a circle and back to the start, so that is one full cycle.

Most electrical signals are AC signals that for the most part look like a sine wave. Here’s a super-primitive ascii art:

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(Yeah, that’s a triangle wave, but the principles still apply, actually)

If you have two signals of the same frequency, that are identical (one goes up and the other goes up at the same time), those are “in phase”. If one signal is slightly ahead or behind the other, then they’re out of phase.

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Since the fundamental AC signal is a sine wave, which is related to a circle, we consider one full cycle to be 360 degrees. That makes sense for countless mathematical reasons, plus the fact that AC power literally comes from a round thing that’s spinning.

So when you have two signals, one of which is behind or ahead of the other, the difference between them is measured in degrees.

Think of singers singing a [round](https://en.wikipedia.org/wiki/Round_(music)), such as “[row, row, row your boat](https://www.youtube.com/watch?v=2d_GLxa4_bg)”. That’s when one set of singers start the song, and then partway through it, another set of singers start singing the same song but behind the first set by a few verses. That’s being out of phase.

If you’re working with radio technology, or signal processing, phase becomes really important.

One more tidbit: a generator with only one set of windings in it will generate one output wave. It’s only really producing the most work at the peaks of the wave. Since that’s not very efficient, they build generators with three sets of windings, all mounted at 120° intervals. This produces three outputs whose signals are — by definition — 120° out of phase with each other. This is why you always see three wires on those high-voltage transmission lines.

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