The nth Fibonacci number is where each number is the sum of the previous 2 numbers. If F(n) is the nth term of the series then we have F(n) = F(n-1) + F(n-2). For the sake of brevity, we won't go too much into the Mathematical proof. However, this is called a recursive formula or a recurrence relation. We need earlier numbers to have been computed before we can compute a later term.