a pathname object.

An implementation-dependent pathname, typically in the working directory that was current when Common Lisp was started up.

a pathname, used as the default whenever a function needs a default pathname and one is not supplied.

 ;; This example illustrates a possible usage for a hypothetical Lisp running on a 
 ;; DEC TOPS-20 file system.  Since pathname conventions vary between Lisp 
 ;; implementations and host file system types, it is not possible to provide a 
 ;; general-purpose, conforming example. 
 *default-pathname-defaults* → #P"PS:<FRED>" 
 (merge-pathnames (make-pathname :name "CALENDAR")) 
→ #P"PS:<FRED>CALENDAR" 
 (let ((*default-pathname-defaults* (pathname "<MARY>"))) 
   (merge-pathnames (make-pathname :name "CALENDAR"))) 
→ #P"<MARY>CALENDAR"

The implementation.