In an op-amp the voltage gain is very high (ideally 10^5) But the output is limited by the power supply voltage of, maybe 12V . So the differential input can not be more than around 0.12 mV for linear operation. So (v+ - v-) is very small, approximately zero. So v+ and v- may be considered equal for all practical purposes. So thare is a virtual short as both input terminals are at the same potential. 'Virtual' because in reality there is no current flow between the two terminals.

as we know there are two terminals in the op amp .. so the condition of virtual short circuit does not happen everytime .. it's present only when there is negative feedback... not in case of positive feedback ... only condition is negative feedback with AB>>1 . i hope this helps.