Definition
Responsive Design is a web design approach that ensures a site or application displays and functions well across different screen sizes, from desktop monitors to tablets and mobile phones.
Importance
With a majority of web traffic now coming from mobile devices for most businesses, an interface that only works well on a desktop screen actively excludes or frustrates a significant share of potential users. Responsive design removes the need to build and maintain entirely separate versions of an application for different devices, which keeps both development cost and long-term maintenance burden considerably lower.
Application
We build interfaces responsively as standard practice, testing across a realiztic range of screen sizes and devices rather than treating mobile as an afterthought checked only at the end of a project. This includes considering how interactions, not just layout, need to adapt between a mouse-driven desktop experience and a touch-driven mobile one, since a responsive layout that hasn't also rethought its interactions can still feel awkward to use on a phone even if it visually resizes correctly.
