Where is the New Oxford American Dictionary stored?

The dictionaries are stored in...

/Library/Dictionaries
~/Library/Dictionaries

In 10.14 (and possibly earlier), the dictionaries have moved to /System/Library/Assets/com_apple_MobileAsset_DictionaryServices_dictionaryOSX/

There is one .asset directory for each downloaded dictionary; for example, my American English dictionary is located at: /System/Library/Assets/com_apple_MobileAsset_DictionaryServices_dictionaryOSX/69b7ab1cf0f75ad16bf6662b0a77fbfd36b7941f.asset/AssetData/New\ Oxford\ American\ Dictionary.dictionary.

UPDATE In macOS 11.5, it seems to be a slightly different path: /System/Library/AssetsV2/com_apple_MobileAsset_DictionaryServices_dictionaryOSX