You can declare a variable using a define form. This takes a variable name, and an expression. It declares a new variable with the given name, and assigned the value of the expression to the variable.

#|kawa:1|# (define binary-kilo 1024)
#|kawa:2|# (define binary-mega (* binary-kilo binary-kilo))
#|kawa:3|# binary-mega

A define typed into the command-line defines a top-level variable.

You can also declare local variables. TODO.