The define command is located under the interactive list.
Figure 1 - Location of the define command.
A function can be defined on the Casio Classpad II using "define" by assigning a function name from the "abc" keyboard (highlighted below):
Figure 2 - The "abc" keyboard that is used to name a function (Func name).
To define a function, start by writing the function in the Main screen and highlight the input before selcting the define function from the interactive list. When defining a function you will be prompted with the following window:
Figure 3 - The pop-up window when using the define command.
Figure 4 - The define command in action.
Figure 5 - Evaluating a defined function at value x = a.
Figure 6 - Solving a defined function.