What is Reactive Programming?

Some say it is just an implementation of the Observable pattern.

The Reactive Manifesto lists 4 attributes of Reactive Systems, but it doesn't exactly define what reactive programming is.

Also while reactive implies that there are entities (e.g models/states) in the system that react to events, I believe that's not enough in order to call a program reactive.

Finally what are Reactive Microservices?

Thanks in advance.