A namestring is a string that represents a filename.
In general, the syntax of namestrings involves the use of implementation-defined conventions, usually those customary for the file system in which the named file resides. The only exception is the syntax of a logical pathname namestring, which is defined in this specification; see Section 19.3.1 (Syntax of Logical Pathname Namestrings).
A conforming program must never unconditionally use a literal namestring other than a logical pathname namestring because Common Lisp does not define any namestring syntax other than that for logical pathnames that would be guaranteed to be portable. However, a conforming program can, if it is careful, successfully manipulate user-supplied data which contains or refers to non-portable namestrings.
A namestring can be coerced to a pathname by the functions pathname or parse-namestring.