What are implicit Objects?

These objects will be automatically instantiated under specific variable names hence the name implicit. Implicit objects are a set of Java objects that the JSP Container makes available to developers in each page.

All the implicit objects maps to something from the Servlet/JSP API. The request implicit object, for example, is a reference to the HttpServletRequest object passed to the service method by the container.

You can use implicit Objects within Scriptlets and Expressions and not within Directives or Declarations.

A good article about Implicit Objects can be found in IBM DeveloperWorks