A symbolic link. This is a file that points to another file, simply by storing the target file's path which may be relative to the location of the SymLink, or absolute. It is also marked as special type of file, just like directories, FIFOs etc.

Using links (whether they be hard or symbolic ones) you can break the strictly hierarchical structure of a FileSystem.

Most file related system calls like open(2) and stat(2) will operate on the referenced file instead of the link itself. Some of them have "SymLink-aware" counterparts that will operate on the SymLink itself instead, eg lstat(2) which stat(2)s the link instead of the linkee.

Comparable to Windows's (GUI) idea of a "shortcut".