reactively

Reactively is an adverb describing the manner in which an action or process occurs as a response to a stimulus or prior event. It denotes behavior that is driven by external input rather than initiated proactively. The form derives from the adjective reactive, which in turn comes from the verb react.

In science and engineering, reactively describes how systems behave when exposed to changing conditions. In chemistry, a substance may act or react under certain conditions; researchers might describe a species as being highly reactive, and experiments may note that reactivity determines outcomes. In engineering and software, reactive systems are designed to respond to inputs or changes in state, often with emphasis on timely or continuous updates. Reactive design and reactive programming focus on data flows, event streams, and the propagation of changes.

In everyday usage, something done reactively happens after an event rather than in anticipation. For example, a company may adjust pricing reactively in response to market shifts, or a device may adjust its display reactively to ambient light.

Related terms include reactive, reactivity, and reactive programming. Reactive, in particular, contrasts with proactive behavior, which aims to anticipate and influence future conditions.