What is the "computer science" term for the practice of assigning a special meaning to one of a type's values. For example a numeric variable called "amount_to_transfer" where the special value "0" means "entire account balance" or a date value "spouse_date_of_birth" where "1/1/1800" means "unmarried". I happen to feel that this is quite a bad "smell", but I'd like to have a name for it, and if possible, some blog post or article about why it's bad and how to fix it.