This global variable is an array related to multi-paged posts. multipage global variable indicates whether the current post is to be shown in multiple pages.
pages is set in the function
setup_postdata($post) defined in [wordpress root folder]/wp-includes/functions.php. A post is split into multiple pages wherever
<!--nextpage--> is inserted. Every element in the
pages array corresponds to the split page. The page to be displayed is determined by the global variable page.
The template function
get_the_content($more_link_text = '(more...)', $stripteaser = 0, $more_file = ''), defined in [wordpress root folder]/wp-includes/template-functions-post.php, uses this global variable to return post content corresponding to the requested page.
Plugin and theme developers can use this global variable to determine number of pages the current post has been split into.
Back to full list of global variables.