From
Phase (waves) - Wikipedia, the free encyclopedia :
"A shift in time is equivalent to a phase shift, for an infinitely long sinusoid. Conversely, a change in the initial phase is tantamount to a shift in time."
Inverting a waveform is the equivalent of a 180 degree change in
initial phase, but you're right in that a 180 degree change in phase could also be caused by the same waveform being delayed for the time it takes half a wave to pass. I'll be clearer next time I talk about changes in phase and changes in initial phase
