Java Runtime Environment (JRE) 8 is an older version of the Java Runtime Environment released by Oracle. JRE 8, also known as Java SE 8 (Java Platform, Standard Edition 8), was a significant release with many new features and improvements. 

Some of the key features introduced in Java 8 include:

1. Lambda expressions: A new functional programming feature that allows you to treat functionality as a method argument.
2. Stream API: A new stream-based API for processing collections of data in a functional-style approach.
3. Date and Time API: A completely redesigned Date and Time API to address the shortcomings of the previous Date and Calendar classes.
4. Default methods in interfaces: Interfaces can now have concrete methods with default implementations.
5. Optional class: A container class that may or may not contain a non-null value.
6. Nashorn JavaScript Engine: A lightweight, high-performance JavaScript engine for Java.
7. JavaFX improvements: JavaFX, the modern Java GUI toolkit, received several updates and enhancements.