A Ghost Page is a Page automatically excluded from all the sitemaps and the shortcode-generated lists, not cached by the WP Super Cache plugin and hidden from crawlers without its path showing up in the 'robots.txt' file. Generally a Ghost Page has few to no inbound links. An example of Ghost Page would be the landing page of a successful PayPal transaction, namely an order confirmation page.
To prevent the indexing of a Ghost Page without resorting to a
Disallow rule that might be used by bad-disposed robots to uncover the Page, SiteTree sends back an
X-Robots-Tag HTTP Header that a trustworthy crawler should take into account. This is also one of the reasons why the plugin hinders the "supercaching" of such a Page.
Flagging a Page as Ghost Page
This can be done by just ticking the 'This is a Ghost Page' checkbox in the editor screen of the Page you want it to be cloaked, and then clicking on the 'Update' button:
Note that neither Posts nor Custom Posts can be flagged as Ghost Pages.