Lisp
McCarthy described a language in which program and data share one list form, and computation is defined by recursive functions over symbols.
Why it matters
Symbolic computation got a working tool, and nearly every AI system of the next thirty years was written in it.
The paper introduced eval as a mathematical definition of the language; Steve Russell noticed eval could be implemented and did so on the IBM 704. Garbage collection, closures and the sameness of code and data entered common use from here. STUDENT, DENDRAL and SHRDLU were all written in Lisp.