X-Content-Type-Options is an HTTP response header used to enhance web security by instructing browsers not to perform MIME type sniffing. When set to nosniff, this header tells the browser to strictly follow the declared Content-Type of a resource, rather than trying to guess it. This helps prevent certain types of attacks, such as Cross-Site Scripting (XSS), by ensuring that files are interpreted only as their specified type. It is commonly used to protect web applications from serving potentially dangerous content in an unintended way.