Jinitiator is a JVM made and distributed by Oracle Corporation

Oracle Corporation

Oracle Corporation is an American multinational computer technology corporation that specializes in developing and marketing hardware systems and enterprise software products – particularly database management systems...

. It allows a web enabled Oracle Forms

Oracle Forms

Oracle Forms is a software product for creating screens that interact with an Oracle database. It has an IDE including an object navigator, property sheet and code editor that uses PL/SQL. It was originally developed to run server-side in character mode terminal sessions. It was ported to other...

 client application to be run inside a web browser. This JVM is called only when a web-based Oracle application

Oracle Applications

Oracle Applications comprise the applications software or business software of Oracle Corporation. The term refers to the non-database parts of Oracle's software portfolio....

is accessed. This behavior is implemented by plugin or an activex control, depending on the browser.

The first two numbers of the version roughly follow the Sun Java numbering convention. It means that for instance Jinitiator 1.3.1.25 is based upon JDK 1.3 or later.

The main reason for Oracle to develop Jinitiator was to support Oracle Forms on the web in earlier releases due to bugs in earlier releases of the JDK. In the latest release of Forms, version 10.1.2.0.2, Oracle is working closely with Sun to phase out Jinitiator, and now officially supports 1.4.2 and 1.5.0 of the Sun JDK. For details, see the latest Client Platform Statement of Direction.

Oracle has announced, that the upcoming release of Forms, version 11, should support JDK 1.6, which will most likely eliminate the need for Jinitiator completely.
On January 25, 2008 Oracle certified Java 1.6u2 for use, eliminating the need for Jinitiator.http://blogs.oracle.com/schan/2008/01/25#a2385