A string (and, in fact, the only string,) of 10 single-digit numbers, d9d8d7d6…d0, such that dn is the number of occurrences of the digit n in the string. It is sometimes written 2100010006 (d1d2d3…d9d0).
It is interesting to note that there is no corresponding string in bases 2, 3, or 6. There are two strings in base 4, 0121 and 0202. In bases 5, 7, 8, and 9, there's one string each: 00212, 0001123, 00010124, 000100125.
In base b, b ≥ 7, there is a regular pattern: "0001", then (7 - b) "0"s, then "12", then (b - 4). These are the only patterns for b between 7 and 50 inclusive, and may be the only type for any base greater than 6.