Lapilli is a size classification term for tephra

Tephra

200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....

, which is material that falls out of the air during a volcanic eruption

Volcano

2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

 or during some meteorite impacts. Lapilli (singular: lapillus) means "little stones" in Latin. They are in some senses similar to ooids or pisoids in calcareous sediments.

By definition lapilli range in size from 2 mm to 64 mm in diameter. A pyroclastic particle greater than 64 mm in diameter is correctly known as a volcanic bomb

Volcanic bomb

A volcanic bomb is a mass of molten rock larger than 65 mm in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. They cool into solid fragments before they reach the ground. Because volcanic bombs cool after they leave the volcano, they do not have grains...

 when molten, or a volcanic block when solid.