Lambda calculus can be called the smallest universal programming language. At its core, lambda calculus is made up of just one "transformation rule" (something called "variable substitution") and just one way to define a function. Each "function definition" has a list of the function's "parameters", which are all of the variables that can be used in that function. Variable substitution is where specific variables in a function are replaced by other values (for example, the value formula_1 could take the place of every spot in a function where the variable formula_2 appears). This is called a transformation rule because it can be used to "transform" lambda expressions by changing around their values.