A self-describing (or autobiographical) number is its own inventory: the digit in position \(i\) (counting from 0) states how many times the digit \(i\) appears in the whole number. Take 1210 — position 0 says "one 0", position 1 says "two 1s", position 2 says "one 2", position 3 says "zero 3s" — and checking the digits 1, 2, 1, 0, it's all true.
There are exactly seven in base 10: 1210, 2020, 21200, 3211000, 42101000, 521001000, and 6210001000. They are a finite, self-referential curiosity — close kin to the look-and-say sequence and to autograms in wordplay.