Lisp

Fortran (q.v.) is the only language in widespread use that is older than Lisp (LISt Processor). Lisp owes its longevity to two facts. First, its core elements occupy a kind of local optimum in the "space" of programming languages, given the resistance to purely notational changes. Recursive use of conditional expressions, representation of symbolic information externally by lists and internally by list data structures (q.v.), and the representation of programs in the same way as data will probably have a very long life.