Jython is a Python interpreter implemented in Java.
Jython is based on Python 2.7. As Python progressed to 3.x already long ago, the supported language features seem to be a bit old fashioned. However the benefits of Jython are that it natively runs on the Java runtime and therefore mixing Java and Python code does not come with any performance penalties.
Jython claims to support almost all of the standard Python library. Libraries using native code like NumPy/SciPy are not supported so far.
Detailed information on Jython can be found in the online Jython book.