differences between variables in mathematics and variables in programming - a variable in programming has a unique, well-defined, _known_ value at any given moment - the value of a variable is set by the programmer to some predetermined constant, or to the result of some process or calculation - the names of variables can be more than one letter - there are exotic programming environments like non-deterministic languages, constraint systems, database query systems, and problem solvers where some of these rules are violated, but they are true for "imperative languages" like Python and the overwhelming majority of languages you would encounter.