Invented by John McCarthy around 1958, Lisp (List Processor) has continued to grow into an entire family of languages.
Since StackOverflow is more about practical programming problems, typically problems will involve actual Lisp dialects or derived languages and their implementations. Problems that are generally Lisp-related may be tagged with lisp. There are many dialects and implementations, but not all are significant for StackOverflow.
Important dialects and related languages are: