Refer to ANSI C84.1 2006 - National Standard for Electric Power Systems and Equipment - Voltage Ratings (60 Hertz).

"A standard specifying a steady-state voltage tolerance for the electric utility at the point of service to be within 5 percent of nominal for non-lighting loads. The standard also specifies steady-state voltage tolerances for acceptable performance of end use equipment of plus 6 percent to minus 13 percent."

There are two ranges (A and B) in the standard. Range A is plus or minus 5% on nominal. Range B is between +6% to -13% nominal. The nominal for Ranges A and B is 120V.

There's more about how the ranges are defined but basically for Range A the number of events outside the range should be few. When voltages outside of Range B occur, "on a sustained basis", the utility is to take corrective actions to bring the voltage back to within Range A.