If you’re reading this text on the WP Glossary website, you’re looking at a responsive website right now. If you resize your window or change the orientation if you’re viewing it on smartphone or tablet, you will notice that the elements fluidly contract, expand and move around depending on the size of the viewport. That means that the website is responsive.
If a website only changes layout at a handful of set screen size breakpoints – once on mobile, once on tablet and once on desktop, for instance – the website is called adaptive rather than responsive. Today, most websites fall under the latter description. That includes most WordPress themes. If you’re browsing for free themes in the WordPress.org theme directory, you can filter the results to only show themes claiming to be responsive through this link.