Wordpress: include content of one page in another

Pages are just posts, with a post_type of 'page' in the database. You can show the content of multiple pages on another page by writing a post query in your pageX template that gets the posts you specify and output them in a Loop.

There are three ways to get post content from the database:

1. get_posts
2. query_posts
3. WP_Query

These links all point to the WordPress Codex. Get_posts and query_posts have an argument available, 'page_id', where you can specify the id of the page you'd like to retrieve and display.

I found this answer posted on the Wordpress forums. You add a little code to functions.php and then just use a shortcode whenever you like.

function get_post_page_content( $atts ) {
        extract( shortcode_atts( array(
            'id' => null,
            'title' => false,
        ), $atts ) );

        $the_query = new WP_Query( 'page_id='.$id );
        while ( $the_query->have_posts() ) {
            $the_query->the_post();
                if($title == true){
                the_title();
                }
                the_content();
        }
        wp_reset_postdata();

    }
    add_shortcode( 'my_content', 'get_post_page_content' );

For the shortcode,

[my_content id="Enter your page id number" title=Set this to true if you want to show title /]

There is not a show_post() function per se in WordPress core but it is extremely easy to write:

function show_post($path) {
  $post = get_page_by_path($path);
  $content = apply_filters('the_content', $post->post_content);
  echo $content;
}

Note that this would be called with the page's path, i.e.:

<?php show_post('about');  // Shows the content of the "About" page. ?>
<?php show_post('products/widget1');  // Shows content of the "Products > Widget" page. ?>

Of course I probably wouldn't name a function as generically as show_post() in case WordPress core adds a same-named function in the future. Your choice though.

Also, and no slight meant to @kevtrout because I know he is very good, consider posting your WordPress questions on StackOverflow's sister site WordPress Answers in the future. There's a much higher percentage of WordPress enthusiasts answering questions over there.