I believe that the original definition of "virtual ground" was
only the zero volts which appears at the negative input pin of an op amp whose noninverting input is connected (perhaps through a resistor) to GND, and has DC feedback to the inverting input.
The term has been used more recently to denote the voltage divider which is used in single-supply op amp circuits, and serves as the bias voltage for the noninverting pin(s) on those op amps. This is not really, IMHO, "virtual ground". It's a bias voltage. The summing vodes of the op amps could be called, I suppose, "virtual bias voltage", or "virtual AC ground".