approach to web design in which a site is crafted to provide an optimal viewing experience—easy reading and navigation with a minimum of resizing, panning, and scrolling—across a wide range of devices (from desktop computer monitors to mobile phones) Wikipedia
and at least one expression that limits the style sheets' scope by using media features, such as width, height, and color. Added in CSS3, media queries let the presentation of content be tailored to a specific range of output devices without having to change the content itself. Mozilla Developer Network