The responsive design deliverable in website development involves creating HTML/CSS templates that ensure optimal viewing and interaction across a wide range of devices, from desktop computers to mobile phones. This approach allows the website to adapt its layout and design to the screen size and orientation of the device being used, providing a seamless and user-friendly experience for visitors. By implementing responsive design, the website can maintain its visual appeal and functionality, regardless of the device used to access it, ultimately enhancing user satisfaction and engagement.