whats a typedef in c++

// typedef [type] [alias]
// Example:
typedef unsigned long int ulong;

ulong someNumber = 158426;

#include <iostream>
 int main(){
	typedef unsigned int ui;
	ui i = 5, j = 8;
	std::cout << "i = " << i << std::endl;
	std::cout << "j = " << j << std::endl;
	return 0;
}