An extensible language is any high-level language that allows its user to modify or enrich its syntax in such a way that a person that does not know the base code cannot tell the customized content from the original. A high-level programming language is a programming language that is more user-friendly, to some extent platform-independent, and abstract from low-level computer processor operations such as memory accesses. ...

Examples of extensible languages are: lisp, forth, Ada 95, Ada 2005 and C++. Lisp is a family of computer programming languages with a long history and a distinctive fully-parenthesized syntax. ... Forth is a programming language and programming environment, initially developed by Charles H. Moore at the US National Radio Astronomy Observatory in the early 1970s. ... Ada is a structured, statically typed imperative computer programming language designed by a team led by Jean Ichbiah of CII Honeywell Bull under contract to the United States Department of Defense during 1977–1983. ... C++ (pronounced see plus plus, IPA: ) is a general-purpose, high-level programming language with low-level facilities. ...

Sources:

Reilly, Edwin D. (2003). Milestone in Computer Science and Information Technolody. Greenwood Prenn, pp. 95. ISBN 1-57356-521-0.