A lacuna is a gap or missing part. If you complain that there's a major lacuna in the bake sale, the lack of brownies is probably to blame.
The noun lacuna means an empty space or a hole where something should be. It's used a lot to talk about missing parts of books or manuscripts, either because lost pages have created a lacuna, or because censors have blacked out or removed parts of them. In the study of anatomy, a lacuna is a hollowed-out place or a cavity within a bone. In Latin, lacuna means "pit or hole," and its plural is lacunae.