Consistenly using the same words to refer to the same concepts avoids confusion. See below word choice recommendations for common computing concepts.
Directories and Folders¶
For containers of files and other containers, use folder. Avoid dir or directory.
If a folder A contains a folder B:
A is the containing folder of B. Avoid parent folder.
B is a subfolder of A. Avoid child folder or sub-folder.
A folder that does not have a containing folder is known as the root folder. Avoid using simply root, except for contexts where the term is already accompanied by a folder icon.