Virtual ground
As an example, a single-ended op amp configured as
an inverting amp (or integrator, differentiator, etc.)
would often tie the + terminal to GND (perhaps via a
resistor, or not). The - terminal is servoed by the amplifier
output via the feedback network to match the + terminal
potential, thus it is "virtual" as opposed to "explicit" even
though (at rest) the two potentials may be identical. The
impedance and current loops, however, are certainly not.