The second category, momentary interruptions, refers to a loss of voltage for periods of less than a cycle to several cycles. They do not last long enough to be called outages, but in effect that's what they are. Voltage sags and swells are the third category. These are variations in the RMS voltage from about one half cycle up to several seconds. They are characterized by changes in the RMS value of the voltage.
Sags refer to a reduction in the voltage, while swells deal with an increase.
Should a sag or swell last for a longer period of time than simply several seconds, we call them undervoltages or overvoltages. This is the fourth category. These disturbances may last indefinitely. The fifth category of power anomalies is harmonic distortion. If the voltage or current waveshape is not sinusoidal, it is described as being distorted.