According to Wikipedia, reactive programming (Rx) "is a declarative programming approach in computers that deals with data streams and change propagation. This paradigm allows users to easily specify static (e.g., arrays) or dynamic (e.g., event emitters) data streams, as well as indicate that an inferred dependence exists inside the related execution model, allowing for automatic propagation of the modified data flow." The word was first used in the IT industry in the 1960s, and since then, much has been written about it.
Unfortunately, as is so frequently the case, the new notion rapidly sparked a slew of misinterpretations that stick to this day. The 2014 Reactive Manifesto, which introduced "reactive systems" and their four sacred principles further twisted things up. In Rx programming, data flows are produced by one component, and the underlying structure supplied by the Rx libraries propagates those changes to other components that have been registered to accept those data changes.