filespec is short for "file specification" and generally means "a string of characters sufficient to specify a particular file within the applicable filesystem", either relative to the current location or in absolute terms. In DOS and Windows, an "absolute filespec" would include either a drive letter ("c:\...") or a hostname ("\\netmachine\sharename\...").
There is a convention for relative filespecs in both Windows/DOS and UNIX/Linux that "." refers to the current directory and ".." refers to the parent directory. Windows/DOS uses back-slash characters ("\") as a separator, while UNIX/Linux uses a forward-slash ("/").
A filespec preceded by a network protocol specifier (e.g. "http://...") is generally referred to as a URL.