I really love org-mode in emacs and want to customize a few things. While reading thru org.el, I'm finding several references to filename but can't find filename using describe-function?

I'm sure there's a simple answer, but I'm just learning elisp and it's not obvious. Any insight into where filename is defined? And/or if it's not a function, what is it?

For example, filename on line 25502:

         (filename (if to-buffer 
         (expand-file-name
          (concat
           (file-name-sans-extension
        (or (and subtree-p
             (org-entry-get (region-beginning)
                    "EXPORT_FILE_NAME" t))
            (file-name-nondirectory buffer-file-name)))
           "." html-extension)
          (file-name-as-directory
           (or pub-dir (org-export-directory :html opt-plist))))))