JRE

=Java Runtime Environment=

It's the Runtime Environment and it's composed of the Virtual Machine and the Java API. But the main element of the JRE is the Virtual Machine which executes .class or .jar files, emulating the JVM instruction set by interpreting it, or using a just-in-time compiler  (JIT) such as Oracle's  Hotspot.

JIT compiling, not interpreting, is used in most JVMs today to achieve greater speed. There are also ahead-of-time compilers that enable developers to precompile class files into native code for particular platforms.

References :

JVM