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A brownout, sometimes also called a sag, is a "dip" in the voltage level of the electrical line. When a brownout occurs, the voltage drops from its normal level to a lower voltage and then returns; in some ways, it's like the opposite of a surge. Most power supplies can handle a reduction in the nominal voltage that it is expecting. The capability of the power supply in this regard is dependent upon its allowable input voltage range. For example, a power supply might be rated for 115 VAC, but may accept anything from 95 V to 135 V. Any reduction of voltage below 95 V for more than a fraction of a second is likely to cause the power supply to either shut down or malfunction.
Brownouts are extremely common, and can lead to mysterious problems you would never blame on the power system. You can sometimes detect a brownout by noticing the lights flickering or dimming; they occur often during heavy load periods such as in the late afternoon on a hot summer day, and also during storms, as the local grid is affected by mechanical failures in adjacent areas. As the world's electric grids become more and more taxed due to increased demand, the incidence of brownouts is increasing.
Brownouts can wreak havoc with computer systems. In many ways, they are worse than a blackout. In a blackout, the power just goes off, but with a brownout the device continues to get power but at a reduced level, and some devices will malfunction rather than failing totally.