The filesystem is presented as a single hierarchy that starts at the directory / and continues downward through an arbitrary number of subdirectories. / is also called the root directory.
The list of directories that must be traversed to locate a particular file, together with its filename, form a “pathname.” Pathnames can be either absolute (/tmp/foo) or relative (book3/filesystem). Relative pathnames are interpreted starting at the current directory. You might be accustomed to thinking of the current directory as a feature of the shell, but every process has a current directory.
The terms file, filename, pathname, and path are more or less interchangeable (or at least, we use them interchangeably in this book). Filename and ...