Simple definition, Java is a programming language. For more detail, Java is an Object Oriented Programming language.
The main purpose of Java invention is “write once and run everywhere”. In fact, a program which is coded by Java can run on many platforms including Windows, OsX, Android, IoT, etc.
Since the introduction of Java in 1991, Java has many versions until now. For example, Java SE (Standard Edition), Java EE (Enterprise Edition), java ME (Micro Edition), etc. Depend on the platform and purpose of using, the customer can chose the which version they want to use.