Assigns the string <value> to the variable <variable>. Single and double quotes have to be doubled like in SQL strings. If this variable does not exist, it will be created. If DEFINE is only called with the name of a variable, the value of the variable will be displayed. As a result of calling DEFINE without parameters, all variables and the assigned values will be listed.
The dot is used as delimiter for variable names (for example, after calling define v=t, the string &v.c1 is replaced by tc1). That's why if you want to add a dot you have to specify two dots (&v..c1 will be evaluated as t.c1)