1. （文件、论点等中对重要问题的）缺漏，脱漏，遗缺
If you say that there is a lacuna in something such as a document or a person's argument, you mean that it does not deal with an important issue and is therefore not effective or convincing.

1. an ornamental sunken panel in a ceiling or dome

Synonym: coffercaisson

2. a blank gap or missing part

Synonym: blank