The filesystem is presented as a single unified 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 (book4/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 one.
The terms file, filename, pathname, and path are more or less interchangeable (or at least, we use them interchangeably in this book). Filename