Extensible language... What?

Is C , C++ or Java extensible languages?

What is an extensible language? I have the definition but no example so I don't understand.

Wikipedia: Extensible programming is a term used in computer science to describe a style of computer programming that focuses on mechanisms to extend the programming language, compiler and runtime environment.

Could you give me examples?

How can you modify the compiler?

Thanks.

Creating new datatypes would make a language extensible?
Then all languages would be extensible.

Except for those languages where creating a new data type isn't possible. (Classic BASIC being one; if you can do it in Fortran I don't remember doing so, etc.)