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. UPS systems can handle a reduction in the nominal voltage due to it’s input voltage window. If the voltage falls outside this window the battery can take over without any disturbance to the load.
Brownouts are extremely common and can sometimes be detected by lights flickering or dimming; often during heavy load periods or severe weather conditions. As demands power increase, so does the risk of brownouts.
Brownouts can wreak havoc with IT loads. 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.