An arithmetic number is one for which the average of the positive divisors is itself an integer. For 14, the divisors are 1, 2, 7, 14; they sum to 24 and there are 4 of them, so the mean is 6 — an integer. So 14 is arithmetic.
Every odd prime is arithmetic (divisors \(1\) and \(p\) average to \((p+1)/2\)). Erdős, Pomerance, and others showed arithmetic numbers have density 1 — almost all numbers are arithmetic — which makes the exceptions (like 2, 4, 8, 9, 10, 12, 16, 18) the interesting ones.