(Or mythically "Lots of Irritating Superfluous Parentheses"). Artificial Intelligence's mother tongue, a symbolic, functional, recursive language based on the ideas of lambda-calculus, variable-length lists and trees as fundamental data types and the interpretation of code as data and vice-versa.
Data objects in Lisp are lists and atoms. Lists may contain lists and atoms. Atoms are either numbers or symbols. Programs in Lisp are themselves lists of symbols which can be treated as data. Most implementations of Lisp allow functions with side-effects but there is a core of Lisp which is purely functional.
All Lisp functions and programs are expressions that return values; this, together with the high memory use of Lisp, gave rise to Alan Perlis's famous quip (itself a take on an Oscar Wilde quote) that "Lisp programmers know the value of everything and the cost of nothing".
The original version was LISP 1, invented by John McCarthy <email@example.com> at MIT in the late 1950s. Lisp is actually older than any other high level language still in use except Fortran. Accordingly, it has undergone considerable change over the years. Modern variants are quite different in detail. The dominant HLL among hackers until the early 1980s, Lisp now shares the throne with C. See languages of choice.
One significant application for Lisp has been as a proof by example that most newer languages, such as COBOL and Ada, are full of unnecessary crocks. When the Right Thing has already been done once, there is no justification for bogosity in newer languages.