The Reactor Pattern is a design pattern that is commonly used in high-concurrency server applications, such as web servers. It is a way of dealing with multiple events or requests that are arriving simultaneously, by using a single thread or a small number of threads to handle them.

In Node.js, the Reactor Pattern is implemented using the event loop, which is a central part of the Node.js runtime. The event loop listens for events, such as incoming HTTP requests or I/O operations, and schedules them for execution by adding them to an event queue.

The event loop then processes events in the queue by calling their event handlers. This allows Node.js to handle a large number of events concurrently, using a small number of threads.

By using the Reactor Pattern, Node.js can handle a high volume of requests with minimal overhead, making it well-suited for building scalable network servers.