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