From AMS Glossary
In a lightning discharge, an increase in channel luminosity accompanied by a rapid electric field variation, itself called an M electric field change.
The M components occur when the channel is already faintly luminous. Downward-moving leaders have not been observed to precede M components. The M components may be confused with branch components, the increases in channel luminosity that occur between each branch and the ground when the upward-propagating return stroke reaches that branch, since the higher branches are obscured by the cloud. The M component is a minor surge of current that reilluminates the channel in a negative cloud-to-ground flash. It may occur within microseconds or up to a few milliseconds of a return stroke. Evidence exists that M components are a result of the sequence of fast in-cloud negative leaders (K changes) contacting a conducting ground channel and renewed ground potential wave that reilluminate the channel.