Which HTML5 element is used to define a navigation menu within a web page?

The HTML5 element used to define a navigation menu within a web page is the <nav> element. Here are the steps to use it:

1. Open your HTML document with a text editor.
2. Locate the section of your HTML document where you want to insert the navigation menu.
3. Start the navigation section by typing <nav>.
4. Inside the <nav> element, you can use list items (<li>) within an unordered list (<ul>) to create the menu items. Each list item can be a link (<a> element) pointing to different sections of your website. For example:

<nav>
  <ul>
    <li><a href="#section1">Section 1</a></li>
    <li><a href="#section2">Section 2</a></li>
    <li><a href="#section3">Section 3</a></li>
  </ul>
</nav>

5. Close the navigation section by typing </nav>.
6. Save your HTML document and open it in a web browser to see the navigation menu in action.

Remember, the <nav> element is not only used for the main navigation menu, but also for other kinds of site navigation, such as a table of contents or an index.