A number has descending digits when each digit is strictly smaller than the one before it: 321, 8531, 9630, 97520.
Unlike ascending numbers, these can use 0 — as the final digit — so the digits are a subset of 0–9 written in decreasing order. The largest example is the pandigital 9876543210.
Together with [[ascending-digits]] they're the two ‘monotone’ digit families; a number that is both ascending and descending can't exist beyond a single digit, since strict monotonicity can only go one way.