Responsive Design
Responsive design is an approach to website design in which the layout adapts automatically to different screen sizes and devices with the help of flexible grids and CSS media queries, instead of maintaining separate desktop and mobile versions.
In practice
Responsive design is the standard approach Google recommends today – in contrast to separate mobile sites such as “m.example.at”, which are now largely obsolete. It simplifies both mobile-first indexing and long-term maintenance, because there is only one codebase to look after.