What is the use of '&' symbol ?

I have been wondering what is the use of '&' symbol.

For example ...

(defun abc ( attr goal &opt prior &aux (tree nil) ) 
...
 )

Sorry for not replying earlier. In Lisp, "&word" is not the symbol "&" followed by "word", it is actually a single symbol name. The "&" has no special significance to the compiler; it is just something that stands out to the human reader.

Here's a list of common naming conventions. http://www.cliki.net/naming%20conventions