Interactive means that programming is a dialog with Lisp. You enter an expression and Lisp computes the side effects (for example output) and the value.
So your programming session is like ‘talking’ with the Lisp system. You work with it until you get the right answers.
Alan Kay (of Smalltalk fame) calls the original definition of Lisp evaluation in Lisp the Maxwell’s equations of programming.
— Rainer Joswig
2010.03.26 Friday ACHK