A user inputs information, such as a mathematical function, composed of variable strings, functions, characters, expressions, etc., into an information input field connected to a function variable processing system. In one embodiment, the function variable processing system breaks down the information into tokens. The tokens are then processed to detect any undefined user definable tokens, e.g., tokens that the user may add and/or change the definitions associated therewith. The function variable processing system generates a display of the undefined user definable tokens along with any associated token definition input fields and/or menus of token definitions. The user may input token definitions using the token definition input fields and/or may select the token definitions from the menus. The function variable processing system associates the undefined user definable tokens with the definitions inputted by the user to convert the undefined user definable tokens to defined user definable tokens. The user definable tokens and associated user inputted definitions are stored and displayed to the user thereof.