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 this occurs your computer could experience such symptoms as the computer locking up, lost data, system crashes, and can also cause hardware in your computer such as the hard drive to fail.
2. Linux command, see the Linux sag command page for further information about this term.