In recreational mathematics, a repunit is a number like 11, 111, or 1111 that contains only the digit 1 — a more specific type of repdigit. The term stands for "repeated unit" and was coined in 1966 by Albert H. Beiler in his book Recreations in the Theory of Numbers.

A repunit prime is a repunit that is also a prime number. Primes that are repunits in base-2 are Mersenne primes. As of May 2025, the largest known prime number 2^136,279,841 − 1, the largest probable prime R_8177207 and the largest elliptic curve primality-proven prime R₁₀₉₂₉₇ are all repunits in various bases.

The base-b repunits are defined as (this b can be either positive or negative)

Thus, the number R_n^(b) consists of n copies of the digit 1 in base-b representation. The first two repunits base-b for n = 1 and n = 2 are

In particular, the decimal (base-10) repunits that are often referred to as simply repunits are defined as

Thus, the number R_n = R_n⁽¹⁰⁾ consists of n copies of the digit 1 in base 10 representation. The sequence of repunits base-10 starts with

Similarly, the repunits base-2 are defined as

Thus, the number R_n⁽²⁾ consists of n copies of the digit 1 in base-2 representation. In fact, the base-2 repunits are the well-known Mersenne numbers M_n = 2ⁿ − 1, they start with