The gap g_n = p_{n+1} − p_n between consecutive primes. By the prime number theorem, the average gap near N is ~ln(N). Cramér's probabilistic model predicts gaps ~ Exp(1/ln N) — each number is prime independently with probability 1/ln N. The actual distribution shows structure: most gaps are even (except 2→3), and twin primes (gap=2) are abundant.