The Collaborative International Dictionary
Lapilli \La*pil"li\, n. pl. [L. lapillus a little stone, dim. of lapis stone.] (Min.) Volcanic ashes, consisting of small, angular, stony fragments or particles.
Wiktionary
n. A sphere shaped droplet of molten lava ejected from a volcanic eruption which falls to earth while still at least partially molten, ranging in size from 2 to 64 millimeters.
WordNet
See lapillus
Wikipedia
Lapilli is a size classification term for tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. Lapilli (singular: lapillus) means "little stones" in Latin.
By definition lapilli range from in diameter. A pyroclastic particle greater than 64 mm in diameter is known as a volcanic bomb when molten, or a volcanic block when solid. Pyroclastic material with particles less than 2 mm in diameter is referred to as volcanic ash.
Usage examples of "lapilli".
Soon, however, lapilli began to come down, then occasional bombs of pumice weighing many pounds.
If they ran outdoors seeking air and freedom, they were met by a thick hail of lapilli that drove them back in terror to the shelter of their homes.
The sifting, seeping flow of ash and lapilli had steadily risen higher and higher, lifting a chained dog with it.
The flood mentioned was probably an avalanche of water, sand, volcanic ashes, and lapilli, such as also on other occasions have occurred on the slopes of the same volcano during periods of torrential rains.
The lapilli form a blanket which soaks up the winter rain and the dew that falls every night, and prevents this trapped moisture from evaporating.