WordNet® 3 (large lexical dictionary of English) is bundled with the app. Dictionaries can be imported from JSON files. How do I get those files? You can take dictionaries available in various formats (DSL StarDict etc.) and convert them using PyGlossary free tool (available on GitHub) to JSON.

Also availble on Windows macOS and Web(as Progressive Web App). The project is built with Flutter and is open source: https://github.com/maxim-saplin/dikt