Reactive Extensions (Rx or ReactiveX) (https://msdn.microsoft.com) is a library for composing asynchronous and event-based programs using observable sequences and LINQ-style query operators. Data sequences can take many forms, such as a stream of data from a file or web service, web services requests, system notifications, or a series of events such as user inputs.

As stated in the preceding definition, these are APIs that allow stream composition using the Observer pattern. It's my duty to introduce you to the Observer pattern before going any further. The following is the definition of this pattern and it's quite intuitive:

The Observer pattern defines a provider (also known as a subject or an observable) and zero, one, or more observers (Subscriber). Observers register with the provider, and whenever a predefined condition, event, or state change occurs, the provider automatically notifies all observers by calling one of their methods. For more information about the Observer pattern, you can refer to this link: https://docs.microsoft.com/en-us/dotnet/standard/events/observer-design-pattern.

Data can flow in a number of forms, such as streams or events. Reactive Extensions lets you convert this dataflow into observables and aids you in programming reactive code.

Rx is implemented in a variety of languages, including Java (RxJava). A full list of implemented languages and more detail on Rx can be found at http://reactivex.io/.