It is the 9-th Fibonacci number F9.
55 is nontrivially palindromic in base 4, base 6 and base 10.
55 is an esthetic number in base 5, base 8 and base 13, because in such bases it adjacent digits differ by 1.
It is a semiprime because it is the product of two primes.
It is a nude number because it is divisible by every one of its digits.
It is a Duffinian number.
55 is an undulating number in base 4 and base 6.
55 is a nontrivial repdigit in base 10.
It is a plaindrome in base 8, base 10, base 12, base 14, base 15 and base 16.
It is a nialpdrome in base 5, base 9, base 10, base 11 and base 13.
It is a zygodrome in base 10.
It is a congruent number.
It is a Kaprekar number, because its square (3025) can be partitioned into two parts whose sum is 55.
It is an upside-down number.
55 is the 4-th centered nonagonal number.
55 is a wasteful number, since it uses less digits than its factorization.
55 is an odious number, because the sum of its binary digits is odd.
The sum of its prime factors is 16.
The square root of 55 is about 7.4161984871. The cubic root of 55 is about 3.8029524608.