What type of lightning occurs entirely inside a thundercloud?

Lightning that occurs entirely inside a thundercloud is called intracloud lightning.

Intracloud lightning, often abbreviated IC lightning, is an electrical discharge between differently charged regions within the same thundercloud. Thunderstorms commonly separate electrical charge as ice crystals, supercooled water droplets, and hail-like particles collide in strong updrafts and downdrafts. When the electric field becomes strong enough, a lightning channel develops between those internal charge areas.

Intracloud flashes are the most common primary type of lightning. They frequently connect the upper anvil region with lower portions of a thunderstorm and may illuminate the entire cloud at night. Because the discharge remains aloft, observers may see a broad sheet-like flash without a visible bolt reaching the ground.

Intracloud lightning differs from cloud-to-cloud lightning, which bridges separate clouds, and cloud-to-ground lightning, which connects a cloud with Earth’s surface. “Heat lightning” is not a separate physical type; it is ordinary distant lightning whose thunder cannot be heard. Forked lightning describes appearance, not where the discharge occurs.