In Latin, “lacuna” means “pool,” from “lacus,” meaning “lake.” The word came into English to refer to holes or gaps in many contexts. In music, a lacuna is a passage where no notes are played. In construction, a lacuna is a gap in the materials, and to a printer, it’s a blank space in a book. There’s a specific anatomical usage, too; lacunae (the plural of “lacuna”) are cavities and depressions in bone. They occur naturally, but they can also be signs of injury or disease.