A repdigit is a positive integer in which all decimal digits are the same. Examples: 1, 22, 333, 4444, 55555, 666666.
Single-digit numbers (1–9) are trivially repdigits. Multi-digit repdigits in base 10 take the form \(d \cdot R_n\) where \(d\) is a digit 1–9 and \(R_n\) is a repunit.
The Beast Number 666 is a repdigit, as is 88888888 (sometimes used in Chinese culture for the auspicious meaning of 8). Repdigits are a strict superset of repunits — every repunit is a repdigit with \(d = 1\).