Defvar and Defparameter

Defvar and defparameter does the same thing as defining a variable but the difference is that in case of defparameter you have to initialize the variable with a value but in case of defvar you can define the variable but you can initialize it later. There is a catch here if you try to use a variable by defining it using defvar without initializing it, it will evaluate to unbound variable, and the unbound-variable slot for that variable will be set.