Given base $b$ a number of the form $d({{b^n - 1} \over {b - 1}})$ for $n > 0$ and $0 < d < b$ is written using using the digit $d$ only, $n$ times in that base and is therefore a repdigit. The term, short for "repeated digit," is credited to Beiler's book Recreations in the theory of numbers, in chapter 11.

When $d = 1$ the resulting repdigit is called a repunit. Only repunits can also be prime (and even then they are rare). No other repdigit can be prime since it is obvious that it is a multiple of a repunit.

In a trivial way, all repdigits are palindromic numbers.