A pandigital number uses all ten digits 0–9 at least once. The smallest is 1,023,456,789; numbers with exactly ten digits that are pandigital are precisely the permutations of 0–9 (with a non-zero lead), of which there are 9 × 9! = 3,265,920.
Pandigital numbers are a recreational-mathematics favourite, especially pandigital primes and pandigital products like 39 × 186 = 7254 (which uses 1–9 once across the whole equation). A related idea, the zeroless pandigital, uses just the digits 1–9.
Because they need at least ten digits, pandigital numbers don't appear among small integers — but every large number can be checked at a glance.