A voltage drop may refer to any of the following:
1. Alternatively referred to as a brownout, dip, sag, voltage reduction, and voltage sag, a voltage drop is any decrease in voltage, typically under one second. Longer reduction in voltage is called undervoltage.
Power line sags often occur during the summer and when too much equipment is plugged into the same circuit. When a voltage drop occurs your computer can lock up, lose data, crash, and may experience hardware failure such as the hard drive failing.