In a typical install, the below is not used or necessary as single items are not being displayed separately.
However, if you are using/linking to single menu item via some permalink or perhaps search results for example, the default url parameter used will be menu_item.
If you wish to override this add the following to your wp-config.php

define('WPPIZZA_SINGLE_PERMALINK_VAR', 'some_string');

somewhere before
/* That's all, stop editing! Happy blogging. */

where some_string is the value you wish to use. (I would suggest to only use lowercase, no spaces and no special characters here)

You have to make sure, that this string is not already used as a url parameter by any of your other plugins on your site or your theme