Over here it's usually caused by a temporary screw up of some sort but here's a definition from Wiki:
Brownout
where the voltage level is below the normal minimum level specified for the system. Systems supplied with three-phase electric power also suffer brownouts if one or more phases are absent, at reduced voltage, or incorrectly phased. Such malfunctions are particularly damaging to electric motors. Some brownouts, called voltage reductions, are made intentionally to prevent a full power outage.

Me too :-)Especially before I get home Monday.