​LAPILLI (pl. of Ital. Lapillo, from Lat. lapillus, dim. of lapis, a stone), a name applied to small fragments of lava ejected from a volcano. They are generally sub angular in shape and vesicular in structure, varying in size from a pea to a walnut. In the Neapolitan dialect the word becomes mpilli-a form sometimes used by English writers on volcanoes. (See Volcanoes.)