Primitive recursion is a method used in mathematics and computer science to define functions based on simpler cases. It starts with a base case, which provides a specific value for a simple input, and then builds more complex cases using previously defined values. This approach allows for the creation of functions that can compute values for natural numbers. An example of a function defined by primitive recursion is the factorial function, denoted as n!. The base case is 0! = 1, and the recursive step defines n! as n × (n-1)!. This method ensures that every function defined is computable and well-defined for all natural numbers.