LISP 2 is a programming language proposed in the 1960s as the successor to Lisp. It had largely Lisp-like semantics and ALGOL 60-like syntax. It is remembered mostly for its syntax, yet it had many features beyond those of early Lisps.
Early Lisps had many limits, including limited data types and slow numerics. Its use of fully parenthesized notation was also considered a problem. The inventor of Lisp, John McCarthy, expected these issues to be addressed in a later version, called notionally Lisp 2. Hence the name Lisp 1.5 for the successor to the earliest Lisp.[1]
Abrahams, Paul W.; Barnett, Jeffrey A.; Book, Erwin; Firth, Donna; Kameny, Stanley L.; Weissman, Clark; Hawkinson, Lowell; Levin, Michael I.; Saunders, Robert A. (1966). "The LISP 2 Programming Language and System"(PDF). Proceedings of the Fall Joint Computer Conference. pp. 661–676.