A binary palindrome has a base-2 representation that reads identically in both directions: 5 is 101, 7 is 111, 9 is 1001, 21 is 10101, 27 is 11011.
Palindromicity is base-dependent — a number can be a palindrome in binary without being one in decimal, and vice versa. Binary palindromes thin out roughly like √n (there are about 2√n of them below n), so they're uncommon enough to be a distinctive property.
The decimal palindromes have their own [[palindrome]] tag; a page may also note which other bases (2–16) it reads palindromically in.
The sequence of binary palindromes is OEIS A006995.