From Glossary of Meteorology
(Also called intracloud flash, cloud-to-cloud flash.) A lightning discharge occurring between a positively charged region and a negatively charged region, both of which may lie in the same cloud.
The most frequent type of cloud discharge is one between a main positively charged region and a main negatively charged region. Cloud flashes tend to outnumber cloud-to-ground flashes. In general, the channel of a cloud flash will be wholly surrounded by cloud. Hence, the channel's luminosity typically produces a diffuse glow when seen from outside the cloud and this widespread glow is called sheet lightning.