As an engineer of some years, I'm slightly puzzled and embarrassed to be asking this question, but I genuinely can't find an answer elsewhere.
Consider a sine wave. Every example has it oscillating about a Y position of zero, which for the sine wave in isolation makes perfect sense.
Suppose it is not oscillating about a Y position of zero though, but about some other point in Y. To generalise, this could be a constant position, or a position moving arbitrarily as defined by some other function. This position would be the centre of the sine wave as a periodic waveform (i.e. the mean - whether arithmetic, least-squares or some other method - as duration tends to infinity). Starting/finishing phase is not relevant.
What do we call this Y offset from zero in the context of the sine wave? Does it have a name?
To generalise further, I suspect this will be general for any periodic waveform and not just a sine wave, but I'm writing a document which is particularly considering sine waves so I would be happy with an answer in that context.
The best I can come up with is "centre" or "offset", neither of which I'm particularly happy with, but I'm not aware of a better term for the concept and I can't find anything online. Hence asking on here!
Note that this is not modulation - the sine wave amplitude, frequency and phase are unchanged.
The term I'm familiar with for this is equilibrium position.