Responsive

Around CSS, responsive design it means flexible layouting.

Around the web it is also compared to reactive design (itself unrelated to reactive programming), where

responsive CSS often means 'will naturally layout differently between desktop and mobile'

reactive CSS often means 'will more intentionally give you a different site between desktop and mobile'

In general, responsive is about

a system responding in a reasonably low amount of time, and/or

not blocking while it's working, and/or

giving feedback / being interactive in some way while it's working