The Reactor can be called the reactive library on top of the JDK. Java doesn’t support Reactive Programming natively, and Reactor is one of the many libraries out there. Reactor comes from the open source group Pivotal and conforms to the Reactive Streams standard. It is built on Java 8 and ReactiveX vocabulary. 

It is worthwhile to note here that, although asynchronous seems to be an important attribute for  Reactive Programming, the Reactor doesn't force you to go asynchronous/synchronous, as it supports both. It depends on the scheduler chosen. That choice is yours. In order to understand the Reactor in a better way, we need to understand Reactive Streams in more detail.