One swing of a pulse away from the baseline and back. A pulse that goes one way only has a single phase and is called monophasic, or unipolar; a pulse that goes one way and then the other has two phases and is called biphasic, or bipolar. The word matters because it is where a pulse count and a frequency part company: counting phases gives twice the number that counting complete pulses does, so a specification that reads double a measured frequency is often counting phases. Clinical electrotherapy keeps the two apart by naming them separately, where phase duration is the time the current flows in one direction and back and pulse duration is the time it flows in both. This is the pulse sense of the word and not the phase angle of a sine wave, which is a different quantity that shares the name. See Bipolar Pulse and Pulses Per Second (pps).
A bipolar pulse leaves the baseline, comes back, and then goes the other way half a cycle later. Both swings induce current, because what the body responds to is the rate the field changes, and the field changes on the way out and again on the way back. The two swings together are one pulse and one cycle, which is why counting swings gives twice the number that counting pulses does. In a measured example the swings sat 50 milliseconds apart inside a 100 millisecond cycle: 10 complete pulses each second, and 20 swings in the same second. See Pulses Per Second (pps).
