Java Virtual Machine (JVM) is a virtual machine that provides a runtime environment to execute Java code or applications. It is an abstraction of an underlying, actual machine, such as the server that the program is running on. JVM converts Java bytecode into machine language and is responsible for allocating memory space. The JVM has two primary functions: to allow Java programs to run on any device or operating system, known as the "write once, run anywhere" principle, and to manage and optimize program memory.

The JVM is detailed by a specification that formally describes what is required in a JVM implementation. The garbage collection algorithm used and any internal optimization of the Java virtual machine instructions are not specified, so as not to unnecessarily constrain implementers.

The JVM has three aspects: specification, implementation, and instance. The JVM specification is a software specification that highlights that its implementation details are not defined within the spec, in order to allow for maximum creativity in its realization. The JVM implementation is the actual software that runs on a computer system, and the JVM instance is an instance of the JVM implementation running a Java program.

In summary, the JVM is a virtual machine that enables a computer to run Java programs as well as programs written in other languages that are also compiled to Java bytecode.