"Reactive"

Reactive websites (layouting)

Reactive websites, responsive design, adaptive websites, and mobile friendly websites, are all different takes on very similar themes -- that theme largely (but not only) being about layouting for different sizes and shapes of screen, and dealing different kinds of input (mouse versus touch).

In this context,

'responsive' often means that the same thing will naturally layout in a different sensible way on a different-shaped or different-sized screen (and you'll get people resizing the browser and saying "see?")

'reactive' (and 'adaptive') often means you get sent a somewhat different site between desktop and mobile.

Reactive programming (state management)

Reactive programming is programming style where state changes are automatically propagated to where they need to go.

This tends to mean

• a relatively declarative style.

• thinking in terms of events (rather than things as ordered by code)

and possibly grouping those events in event streams

a fairly asynchronous system of execution (in terms of event ordering)

It's also a lot closer to functional programming than people seem to realize.

This concept is used anywhere from

to the hidden model of GUI updates

see also e.g. Vue, Svelte, where what looks like regular JS

program language design

that is, languages with added features, like coroutines;

a fully reactive languages have proven to be useful for specific purposes but not as a generic language