La différence entre responsif et adaptatif

Responsive sites and adaptive sites are the same in that they both change appearance based on the browser environment they are being viewed on (the most common thing: the browser’s width).
Responsive websites respond to the size of the browser at any given point. No matter what the browser width may be, the site adjusts its layout (and perhaps functionality) in a way that is optimized to the screen. Is the browser 300px wide or 30000px wide? It doesn’t matter because the layout will respond accordingly. Well, at least if it’s done correctly!
Adaptive websites adapt to the width of the browser at a specific points. In other words, the website is only concerned about the browser being a specific width, at which point it adapts the layout.
Geoff Graham, The Difference Between Responsive and Adaptive Design

Une définition, et des explications techniques plus poussées. Quelques points essentiels à retenir :

Pourquoi vouloir faire à ce point la différence ? Parce qu’en terme de conception, cela a beaucoup d’implications !