Q.What do you mean by virtual ground with respect to an op-amp?

The virtual ground (or virtual short) is one of the most useful ideas for analysing op-amp circuits with negative feedback. It follows directly from two ideal-op-amp assumptions: the open-loop gain is so large that the voltage between the two inputs is essentially zero, and no current flows into either input.

Consider an inverting configuration where the non-inverting $(+)$ terminal is connected to ground. Negative feedback drives the output to whatever value keeps the difference between the inputs near zero, so the inverting $(-)$ terminal is held at almost exactly $0\ \text{V}$ — the same potential as the grounded $(+)$ terminal — even though the inverting node is never physically wired to ground. That node therefore behaves like a ground for voltage purposes; hence "virtual ground."

Two consequences make circuit analysis simple:

• The inverting node sits at $0\ \text{V}$, so the full input voltage appears across the input resistor and the full output appears across the feedback resistor. …