Phase refers to a time difference between voltages or currents which typically have similar waveforms, and is usually given in degrees of angle.
It is also used to refer to the wires carrying these voltages and or currents. Gets confusing, as in "what is the phase (angle) of the third phase?".
So in single phase, there is one current only, so phase is actually not very relevant. The wires are often referred to as a "phase" anyway. See next def.
Three phase power wiring has three relevant currents, which are nominally 120 degrees apart in angular phase. It is easy to see, hard to explain in words.
But if you imagine 3 sine waves evenly spaced on a horizontal line, so that there are three separate "up loops" on the drawing, and in the fourth position the first line repeats, you have it. The line represents time increasing to the right.
Single phase would have only one sine wave drawn.
The currents and associated wires are also referred to as "phases".
So the first phase, the second, etc, have a "phase (angle) difference of 120 degrees between them, or 360 total, which is why the first one then repeats.
You can see that single-phase has places where it is zero. But with three phase, one or another of them is always near maximum. By imagining the drawing to represent power transfer, you can see why 3 phase is smoother.
To confuse the issue more, single-phase is derived from 3phase, by splitting out a "phase" (wires). So that accounts for some of the duplicate use of the word.
Clear as mud?
