Until recently (although I'm not sure how recent), macOS shipped Dictionary.app with dictionary definitions built in.
As of macOS Mojave, this is no longer the case. The first time you open Dictionary.app, you are prompted to download definitions from the internet.
How can I transfer a copy of these definitions to a fully offline computer? I can't seem to figure out where they are stored.
A way to capture the original .pkg file from Apple's servers (assuming this is how the downloads are distributed) would also be an acceptable answer.
Per answers below, I now know that dictionaries are stored as .dictionary files in /System/Library/Assets/com_apple_MobileAsset_DictionaryServices. However, moving them to this folder on a new machine does not cause them to be recognized in Dictionary.app.
Interestingly, if you open one of these .dictionary files in Dictionary.app, you get a message saying the dictionary could not be imported, because the "folder" format is not supported. Which begs the question, what format would be supported...