Actually, I wasn't confused. I just disagree. I could be wrong, of course.@gootee
You may be confused by the DC and AC coupling options on oscilloscopes. These aren't the same as DC and AC voltage reading. DC voltage has always a single value and stable polarity. Any fluctuating voltage is considered AC and is made up from various frequency components (see Fourier analysis) and possibly from a DC component (average value).
I always thought that AC meant that the current waveform had to cross through zero, so it would change (i.e. alternate) directions, making it "Alternating Current".
In other words, DC is anything with only one polarity and AC requires both polarities.
If a voltage or current was constant, or fixed, we called it "constant DC", or "time-invariant", or "fixed".
I think it would be difficult to define and use AC and DC, your way. What kind of "fluctuations" would be counted, in order to call something "AC"?
When my battery voltage changes (fluctuates) from the fully-charged voltage to the discharged voltage, or when it merely dips momentarily under a heavy transient load, is it an AC battery voltage? What if I make the battery's voltage dip repeatedly, periodically, even sinusoidally? AC battery voltage. Hmmm.
What about bridge rectifier output voltage? I think it should be called "fluctuating DC voltage", or "time-varying DC voltage". I think that since it does not alternate polarities, it is "DC".
And then there would be cases like in an audio power amplifier, where the power rail voltages are almost constant DC (as constant as we want, at least), but the currents, which make the output signal, can be very dynamically time-varying, even sinusoidal, but still always only go in one direction. So then you would say that we have a DC voltage and its AC current in the same conductor?
Here it is on Wikipedia:
http://en.wikipedia.org/wiki/Direct_current
It's not entirely clear-cut. They do say:
"Direct current (DC) is the unidirectional flow of electric charge."
and
"Although DC stands for "direct current", DC often refers to "constant polarity". Under this definition, DC voltages can vary in time, as seen in the raw output of a rectifier or the fluctuating voice signal on a telephone line."
but they also have:
"The DC solution of an electric circuit is the solution where all voltages and currents are constant." and some even stricter statements about DC being "constant".
In the end, I don't think you can call something "AC" unless its current reverses direction.
I would probably prefer to see terms like "AC component" and "DC component" if there is a time-varying waveform that is of only one polarity, e.g. a sine with a large-enough DC offset.
I do tend to think of any time-varying waveform as AC, even if it is technically DC. But I almost never call anything "AC" or "DC", except for primary power sources.
I guess it's all a bit too academic, when time-varying signals are involved. "AC" is probably more of just a power-grid type of term. We eventually just looked at everything as general functions of time or frequency and didn't usually care what it was called.
But, of course, being just a wee bit overly-pedantic, I don't mind dessecting and debating almost anything.
Regards,
Tom