You are allowed to define your own variables and functions. As a name of a variable or function can be a name consists of letters, digits or an underline character (only the first character must be a letter). From version 0.8.3 names are case-sensitive that means 'name' is not the same as 'NaMe'. For example these are correct names: a, b, c3, MyName. During defining variables and functions you can call another variables and functions.
Functions can get maximum 9 parameters. You have to specify how many parameters your function takes and you can refer to them by using the variable 'x' with a number from 1 to 9 ('x' without a number means 'x1'). For example you can define a function 'xy' as 'x1*x2' (of course without apostrophes), set the 'parameters' to '2' and then put into the edit box expression 'xy(10;20)', the result will be '200'. Note that we use a semicolon to separate arguments in the function.
More complex example: myfunction(x) which returns sin(x) when the x is negative, 0 when the x is 0, and cos(x) when the x is greater than 0:
myfunction(x)='if(x==0; 0; if(x<0; sin(x); cos(x)))'