What is the term for an instruction within an HTML page that directs the Web server to perform an action?

The term that describes an instruction within an HTML page directing the web server to perform an action is known as a server-side include. This directive allows for the inclusion of other files within the HTML document processed on the server before being sent to the client. It enables functionalities like inserting header or footer content or other reusable code snippets dynamically.

Using server-side includes allows for better management of code and content, as it avoids redundancy by letting developers insert common elements across multiple pages without duplicating code. This is particularly useful in maintaining a consistent user experience and managing updates efficiently.

Other options, while related to HTML and web development, do not serve the same purpose. A client-side script runs in the user's browser and is executed after the page loads, rather than instructing the server to perform an action. A document type declaration informs the web browser about the version of HTML being used, ensuring that the page is rendered correctly but does not direct any actions on the server. An HTML comment is used to provide notes or explanations within the code that do not appear in the rendered output, serving only as documentation for developers.