Reactive system is defined in the reactive manifesto at http://www.reactivemanifesto.org/. The creators of the manifesto realized that with the change of technology, new system patterns will need to be developed in enterprise computing to leverage the new technology and yield better outcomes. The manifesto envisions systems that are:

• Responsive
• Resilient
• Elastic
• Message-driven.

The first three features are user values; the last one is more of a technological approach to get the values.