Just learned that the question "what is java" is on place 3 of Google's Zeitgeist ranking 2008 for "what is" questions - right after "what is love" (first place) and "what is life" (second place).
Indeed a good question. Let's take a look a Google's top hits to this question:
Java is a computer programming language. It enables programmers to write computer instructions using English based commands, instead of having to write in numeric codes. It’s known as a “high-level” language because it can be read and written easily by humans. Like English, Java has a set of rules that determine how the instructions are written. These rules are known as its “syntax”. Once a program has been written, the high-level instructions are translated into numeric codes that computers can understand and execute. (Source: http://java.about.com/od/gettingstarted/a/whatisjava.htm)
A high-level programming language developed by Sun Microsystems. Java was originally called OAK, and was designed for handheld devices and set-top boxes. Oak was unsuccessful so in 1995 Sun changed the name to Java and modified the language to take advantage of the burgeoning World Wide Web.
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities. Java applications are typically compiled to bytecode (class file) that can run on any Java Virtual Machine (JVM) regardless of computer architecture. (Source: http://en.wikipedia.org/wiki/Java_(programming_language))
Java is a technology that allows software designed and written just once for an idealized "virtual machine" to run on a variety of real computers, including Windows PCs, Macintoshes, and Unix computers. On the web, Java is quite popular on web servers, used "under the hood" by many of the largest interactive websites. Here it serves the same role that PHP, ASP or Perl might, although traditionally Java has been used for larger-scale projects.
What is Java? Java allows you to play online games, chat with people around the world, calculate your mortgage interest, and view images in 3D, just to name a few. It's also integral to the intranet applications and other e-business solutions that are the foundation of corporate computing.
As you see, it's not an easy question to answer!