It's called 'unlink' because it removes the directory entry (file name), which can be considered a 'link' to the actual file (the data).

Note that (on Unix) multiple file names (hard or symbolic links) can point to the same file.  The unlink system call (or the user command of the same name) deletes the name, the file itself is only deleted if that name was the last (hard) link/reference to the file.