A nialpdrome is a number whose hexadecimal digits are in nonincreasing order. The first few are 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 32, 33, 34, 48, 49, 50, ... (Sloane's A023771), corresponding to 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, 10, 11, 20, 21, 22, 30, 31, 32, ....
Digit, Hexadecimal, Katadrome, Metadrome, Plaindrome
Sloane, N. J. A. Sequences A023771 in "The On-Line Encyclopedia of Integer Sequences." http://www.research.att.com/~njas/sequences/.