la·pil·lus (l?-p?l??s)n. pl. la·pil·li (-p?l???) A small, solidified fragment of lava.[Latin, diminutive of lapis, stone.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.