What is a directory in the sense of a computer files?
Directories and folders are interchangeable terms. In the days
before Windows (known as the "DOS" days), "Folders" were called,
"Directories." To explain what a directory was, it was said to be
"like a folder" which held documents (files) and sometimes
envelopes (sub-directories) which could also hold other envelops
and/or documents etc. etc. At some point Microsoft started
referring to "Directories" as "Folders" supposedly to make it
easier to understand for those new to computers.
Computer files are always found in a named Directory. If a file
isn't in a named directory, it is said to be in the "Root"
directory i.e "In the root directory of the D: drive."
Further, if a sub-directory/folder is in another directory, it
is the "child" of the "parent" directory.