Reactive Programming is a "chameleon"! :-) It can be understood as "a style of micro-architecture involving intelligent routing and consumption of events, all combining to change behaviour"¹; or it can be defined as "a programming paradigm oriented around data flows and the propagation of change"². It has something to do with the Reactive Manifesto. Functional programming comes to mind when thinking about reactive programming. And it is a software development field where a lot of interesting progress is being made! Lots to discuss here!

Functional programming paradigm and languages are usually associated with core features like pure functions and immutable data (structures)¹, and other properties like referential transparency, higher-order (first order) functions, lazy evaluation, and recursion². They arguably provide programmers with better tools and abstractions to deal with difficult actual problems that involve concurrency, parallelism, big data, etc.