A unit fraction is a positive fraction with one as its numerator, and a whole number for the denominator. It represents one part of a whole divided into a number of equal parts. Examples of unit fractions include 1/2, 1/3, 1/4, and so on. Unit fractions are often introduced earlier than other kinds of fractions in mathematics education because of the ease of explaining them visually as equal parts of a whole.

Multiplying two unit fractions produces another unit fraction, but other arithmetic operations do not preserve unit fractions. In modular arithmetic, unit fractions can be converted into equivalent whole numbers, allowing modular division to be transformed into multiplication. Every rational number can be represented as a sum of distinct unit fractions, and these representations are called Egyptian fractions based on their use in ancient Egyptian mathematics.

Unit fractions are commonly used in fair division, and this familiar application is used in mathematics education as an early step toward the understanding of other fractions. In geometry, unit fractions can be used to characterize the curvature of triangle groups and the tangencies of Ford circles. Unit fractions are also common in probability theory due to the principle of indifference.